I did some additional research regarding the German surrender to 21 Army Group and came across a few items of interest.
From "Beetle: The Life of General Walter Bedell Smith" by D.K.R. Crosswell
(p.915-16)Friedeburg offered the surrender of German forces facing the Russians in Mecklenburg. "Nothing to do with me," Monty said. He stated his willingness to accept the surrender of any German soldier entering his lines, but Montgomery wanted the capitulation of German forces in western Netherlands, Friesland and the Frisian Islands, Helgoland, Schleswig-Holstein and Denmark. After being harangued by Montogomery on the mercilessness of the Nazi regime and hearing him reject any appeals for safeguarding the lives of German citizens - according to Friedeburg, his reason for coming - the German admiral said he possessed no authority to surrender German forces but requested permission to discuss the field marshal's demands with Dönitz and Keitel, chief of staff of OKW. Friedeburg asked for forty-eight hours, and Montgomery gave him twenty-four.
Dönitz had already decided to surrender unconditionally to the British, but Friedeburg's mission had bought another day. Although refusing to surrender on all fronts, in a demonstration of good faith, Dönitz suspended u-boat activities and released King Leopold of Belgium. At the appointed time, Friedeburg returned to Montgomery's headquarters and signed the "Instrument of Surrender." Typical of Montgomery, the document read that the Germans surrendered to the person of the "C-in-C 21 Army Group." (Montgomery Diary. "Notes on the Campaign in Northwest Europe" p 525)
From "The Day the War Ended: May 8, 1945 - Victory in Europe" by Martin Gilbert
(pp 60-61)The Grand Admiral, the General and the Major had come to surrender three German armies then facing the Russians. Montgomery rejected their offer. Surrender of the forces facing the Russians, he said, must be made to the Russians, and to the Russians alone. They could surrender to him only those armies facing the British; that is, all the German forces in Holland, north-west Germany and Denmark. Montgomery told them, as he reported to London: 'If they refused to agree as above, then I would go on fighting and a great many German soldiers and civilians would be killed. (Telegram to Field Marshal Alan Brooke, quoted in Nigel Hamilton, Monty, The Field-Marshal, 1944-1976, p 505) Major Trumbull Warren, a Canadian officer at Montgomery's headquarters, recalled: 'He told them to look at the maps that showed where we were and where they were. He told them that we had tremendous strength pouring into Germany on the ground and that we had sufficient aircraft for 10,000 bombers, day and night. (Trumbull Warren, 'The Surrender of the German Armed Forces' quoted in Nigel Hamilton, Monty, The Field-Marshal, 1944-1976, p 506)
The German officers crossed back through the lines and returned to Flensburg, where they put Montgomery's condition to Grand Admiral Dönitz and Field Marshal Keitel. At half past five on the afternoon of May 4 the four German officers returned. An hour later they signed the instruments of surrender of all forces facing Montgomery's. It would come into effect at 8 o'clock on the following morning. Listening at that moment to German radio traffic was Norman Cohen, a radio operator attached to Tactical Headquarters Staff, British 2nd Army. He heard, and translated on his note pad, a message from Admiral von Friedeburg to Admiral Dönitz: 'Have signed conditions, including shipping same zones. The cease-fire will take effect from 8 o'clock on May 5.' (The text of the intercepted message was published by Alice Brickach in a letter to the Narragansett Times, 13, July 1944) Cohen kept the note pad as a souvenir.
All German forces in Holland, Denmark and northwest Germany would lay down their arms. German merchant shipping off the North Sea Coast would likewise surrender. ...
I have to question the above author's interpretation regarding the North Sea Coast, as by this time the majority of the German shipping was limited to the waters of the Baltic Sea.
And from Memoirs: Ten Years and Twenty Days by Karl Dönitz
(p.453)Since April 26 the British had been occupying a bridgehead near Lauenburg on the east bank of the Elbe. From it they launched an attack on May 2 and over-ran the weak German defences. Very swiftly British troops and tanks had thrust forward as far as Lübeck. At the same time, a little farther to the south, the Americans crossed the Elbe and reached Wismar without opposition. Thus from the Baltic to the Elbe the British and Americans were standing astride the roads leading from Mecklenburg to Holstein, which were choked with columns of refugees and the retreating troops of the Vistula army. The gateway to the west was no longer open, and it depended upon British acquiescence whether or not the troops and refugees would be allowed to escape from the pursuing Russians into the British zone in Schleswig-Holstein. It had been solely for the purpose of keeping this gateway into Schleswig-Holstein open for the refugees that the fight against the western Allies had continued on the Elbe. Now that Schleswig-Holstein was in British hands, there was no point in persisting. I therefor gave the orders that surrender negotiations, in accordancewith our prepared plan, should forthwith be initiated. Friedeburg was to go in the first instance to Montgomery and offer to surrender north-west Germany to him. Then, when that had been accepted, he was to go on to Eisenhower and offer the surrender of the rest of the western theatre.
The Officer Commanding Hamburg District was directed by Supreme Headquarters to send an officer with a flag of truce to the British at eight o'clock on the morning of May 3 to offer the surrender of Hamburg and at the same time to inform the British that a delegation under Admiral was on its way.
(p. 454)Towards evening the air attacks stopped. I told Freideburg to meet me at the Levensau bridge over the Kaiser Wilhelm canal near Kiel. Graf Schwerin-Krosigk and I reached the rendezvous without incident. My instructions to Freideburg were that he was to offer Montgomery the military surrender of the whole of north Germany and at the same time invite the Field-Marshal's special attention to the problem of the refugees and troops in retreat on the eastern boundaries of the area occupied by the British. He was in particular to do his utmost to ensure that the evacuation and withdrawals by land and sea were not adversely affected by the surrender, but would be allowed to continue. He took his leave of us in the darkness, accompanied by our best wishes for his success.
(p. 457-58)Shortly before midnight Friedeburg returned from his negotiations with Montgomery and reported at once on the salient features of his conference. The Field-Marshal, he said, had not rejected the proposed separate surrender - had not, that is, demanded simultaneous unconditional surrender on all fronts including the Russian.
On the morning of May 4 Friedeburg submitted his detailed report in the presence of Schwerin-Krosigk, Keitel and Jodl. Montgomery, he informed us, was prepared to accept separate surrender of north Germany, but he demanded that Holland and Denmark should be included. He, Friedeburg, had replied that he had not the authority to agree to this, but that he was sure that I would agree. Montgomery had also demanded, he continued, the simultaneous surrender of all warships and merchantmen. This, of course, affected the vital question of our refugee evacuation organization, and Friedeburg had therefore explained our problem and our anxiety to bring as many as possible into the western territories and save them from falling into the hands of the Russians. Montgomery had replied that he would not prevent individual soldiers from surrendering, but that in no circumstances could he accept the surrender of formed bodies of troops. As regards refugees he had refused to give any guarantees, for, he said, the question at issue was that of a purely military surrender and civilian affairs did not come into it; he had, however, added that he was 'no monster'. Montgomery had made the further stipulation that there should be no demolitions and no sinking of warships within the area to be surrendered. Friedeburg had then asked to be allowed to report to me, since he did not have the authority to accept some of the demands.
So much for Friedeburg's report. As far as an extension of the surrender to include the Netherlands and Denmark was concerned, both Schwerin-Krosigk and I were only too pleased at the prospect of 'getting these countries off our hands' and seeing them handed over in an orderly manner as soon as possible.
The demand that we should surrender our ships disturbed me gravely. It would mean the end of the evacuation by sea of troops and refugees to the western territories. From Frideburg's report on Montgomery's attitude towards this question, however, I had the impression that it might be possible to allow those ships already at sea to continue their voyage westwards. But the wounded, refugees and troops aboard them would have to be landed at Danish ports. The arrival of some 300,000 Germans would put a great political and administrative strain on Denmark's slender resources. Quarters, food and medical attention for so vast a number of foreigners, and hostile foreigners to boot, would present a problem of very great difficulty. But these were the disadvantages which we perforce had to accept with good grace. ...
We could not endanger our primary object, namely, to make separate capitulations in order to save human lives. The officers of Supreme Headquarters on the other hand were of the opinion that to hand over weapons, and particularly warships, the most strikingly outward and visible of armed strength, would be a violation of the tenets of military honour.
(p. 459)On the morning of May 4 I gave Freideburg full authority to accept Montgomery's conditions. He flew back to British Headquarters with instructions that, as soon as the formalities of the separate surrender to Montgomery had been completed, he was to fly on to Eisenhower in Rheims, and offer him in the same manner the separate surrender of our forces in the American sector.
Freideburg's report of May 4 had come as a great relief to us. The first step towards a separate surrender to the West had been accomplished without our having been forced to abandon German soldiers and civilians to the mercy of the Russians.
From the above quoted sources, it appears to me that when F.M. Montgomery agreed to the "Instrument of Surrender of all German Armed Forces in Holland, in Northwest Germany Including all Islands, and in Denmark; May 4, 1945 ", it was for a separate surrender of only those forces named in the document and in particular, didn't take into consideration the Soviets or Americans as evidenced by his reply to Friedeburg's offer of surrender of the German forces facing the Soviets in Mecklenburg: "Nothing to do with me,"