Guarantee by Britain was not about allowing Poland to take Danzig and territories east of the Oder-Neisse Line, it was not in the original document, so let's stick to facts.
I respectfully suggest that you are not acquainted with the full facts in relation to this issue, and of their implications. I suggest you read the text of the Anglo-Polish agreement on military co-operation of 6 April 1939.
That agreement did not only state that Britain would come to the aid of Poland if that country sent its armed forces into action against a DIRECT threat to its independence, such as a military invasion. It also stated that it would come to Poland's aid if that country resisted an INDIRECT threat.
The agreement did not define what an indirect threat was, but the British Government informed the United States Government that it would accept anything that the Polish Government claimed to be an indirect threat to the independence of Poland. It also acknowledged to the United States Government that the guarantee it had given to Poland essentially gave that country the power to decide whether Britain would go to war against Germany.
By June 1939, Chamberlain had openly stated in the Commons that Britain accepted Danzig as vital to the independence of Poland, and therefore any attempt by Germany to take control of it would be regarded as a threat to that independence, and would trigger the action called for in the Guarantee and the Anglo-Polish Agreement. That represented a remarkable turnaround, since Britain had never previously regarded Danzig as vital to Poland, and had even favoured its eventual return to Germany; the difference now was that the Danzig issue provided a plausible casus belli against Germany, much like Belgium in 1914.
To be sure, neither the Guarantee nor the Anglo-Polish Agreement on Military Co-operation said anything about giving German territory to Poland, but that is not the point. The point is that those nationalist forces in Poland that desired westward expansion to the Oder-Neisse Line, essentially the opponents of Pilsudski's regime, saw in the Guarantee and the Agreement the possibility of waging a successful war against Germany that would permit them to achieve that aim.
Westward expansion to the Oder-Neisse Line had long been an ambition of certain Polish nationalist groups, particularly of the National Democrats and other opponents of Pilsudski, although they had not been able to achieve it due to the opposition of the Great Powers. That is why one of the first things Sikorski did when in 1940 he transferred to Britain the seat of the Polish Government-in-Exile headed by him, was to declare the expansion of Poland to the Oder-Neisse Line as a major Polish war aim, and to ask the British Government to endorse that aim. At that time the British Government declined to give that endorsement, since its aim was to eliminate German military and industrial power, not to enhance Poland, a country it did not particularly trust or like.
Back then in April 1939, when Hitler laud and clear demanded Danzig during his public speech of April 28, what guarantees Poles had, that situation won't repeat itself and Poland won't be "peacefully" conquered like Czechoslovakia, after agreeing to Hitler's demands?
Are you aware that 28 April comes after 6 April? I take it you realise that time moves forward, not backward.
Hitler's speech of 28 April 1939, denouncing the German-Polish Declaration of Non-Aggression of January 1934, was made in a completely changed context, where on 6 April Poland had entered into a military agreement with Britain aimed against Germany, as a precursor to a full alliance. If Poland had not accepted Britain's offer of an anti-German alliance, as Romania for example did not, there is no reason to believe that Hitler would have turned against that country, or ordered the preparation of an invasion plan.
