I wouldn't agree with this theory. In all major (and the most important) battles of the campaign armour played the main and the decisive role. Bzura and Kampinoska Forest (1., 4. Panzer divisions, 2., 3., 1. Leichte divisions, 29. Motorisiert division), Piotrkow Trybunalski - Tomaszow Mazowiecki (1. and 4. Panzer divisions), Ilza - Radom - Kielce (1., 2., 3. Leichte, 29., 13. Motorisiert), Pszczyna or combats for Upper Silesia in general (5. Panzer division), combats for the Mlawa line and later for Rozan (Panzer-Division "Kempf"). Armoured-mot. forces also participated in the siege of Warsaw (for example the failed assault of 4. Panzer-Division on 09.09.1939) and in the siege of Modlin.
Even during the battle of Lwow (and battles west and north of Lwow), German 5. and 2. Panzer divisions participated.
In almost all of these battles mentioned above, armoured-motorized forces were the decisive factor of victory.
Armoured and motorized divisions also suffered heavy casualties in comparison with casualties suffered on average by each of infantry divisions. In general armoured-motorized (fast) forces suffered 29 - 30% of Heer human casualties in Poland as well as 100% of German armoured vehicles casualties in Poland, while being "only" 25% of German forces.
10. Army was the most important German army during the Polish campaign. It consisted of 14 divisions including as many as 7 armoured-motorized divisions, concentrated in three armoured-motorized Corps - XVI., XV. and XIV. - which used to cooperate with each other. And around 50% of men casualties of this army were suffered by fast forces. At the same time 10. Army was the army which suffered the heaviest men casualties out of all German armies involved.
but by far the bulk of the fighting and effort is made by infantry/Gebirgs divisions, e.g. Bzura, the advance on Lvov.
During the battle of Bzura infantry played an important role, here I agree. But the decisive role was once again played by armoured divisions, mainly 1. and 4. Panzer (but also during the later phase of the battle - 1., 2. and 3. light, 29. mot.). It was the German armoured assault on 16.09.1939 and 17.09.1939 which, despite being a failure, caused the decision of gen. Tadeusz Kutrzeba to stop the battle and withdraw behind the Bzura river to the Kampinoska forest.
the bulk of the fighting and effort [...] e.g. [...] the advance on Lvov.
The German advance on Lvov - as well as its alleged importance - is mythologized by German sources.
During the advance on Lvov German 1. Gebirgs-Division almost didn't encounter any resistance because simply it didn't have any strong Polish units in front of it (only when it reached Lwow, hell started). But during the same time other forces of the German 14. Army (including 2. and 5. Panzer division as well as 4. Leichte division) were involved in heavy purchase (from the German point of view) and slowing down (from the Polish point of view) fightings, including combats against Polish 10. Motorized Brigade of colonel Stanislaw Maczek. Later 2. Panzer & 4. Leichte divisions were involved (and played an important role) in the 2nd biggest battle of the campaign - Tomaszow Lubelski.
the dash across the Corridor
The dash across the Corridor was not only the dash but also a battle, a quite important battle because as the result Poland was cut off from its only access to the Baltic sea and from its only important port - Gdynia. Also after the dash across the Corridor Guderian's XIX. Panzer-Korps was involved in heavy fightings in North-Eastern Poland (Lomza, Wizna, Andrzejewo, Zambrow, Fortress Brest-Litovsk, Wlodawa, Kobryn, finally a failed attempt to capture Chelm Lubelski). As the result Guderian's XIX. Panzer-Korps suffered heavy casualties, especially 20. Motorisiert-Division.
Only on 15.09.1939 and 16.09.1939 during combats on the section of the Northern Gate of Fortress Brest-Litovsk, 20. Motorisiert-Division lost 134 KIA (including 7 officers KIA) and 240 WIA. During the whole campaign in Poland this division lost at least 24 officers KIA, thus I estimate its casualties as at least 455 - 460 KIA and 815 - 825 WIA.
e.g. the advance on Warsaw
But this advance on Warsaw was full of battles, very important battles. The most important of them was the battle of Piotrkow Trybunalski - Tomaszow Mazowiecki (04.09.1939 - 07.09.1939), during which German forces (XVI. Panzer-Korps - two armoured and two infantry divisions) broke through the positions of Polish 19., 29. and 13. Inf.Div.
Piotrkow - Tomaszow is often considered by Polish historians as the most decisive battle of that campaign - not the battle of Bzura. It was also a very bloody battle for both sides. Germans lost over 130 tanks during that battle.
I already knew the horse, not the panzer or motor vehicle, was the German Army's biggest means of transport.
Even in German infantry divisions the horse was not the most important mean of transport, at least if comparing with Polish infantry divisions. In Polish active service infantry division there were 76 cars & trucks, 20 motorcycles and 6939 horses. In German infantry division - between 1009 cars & trucks (this includes 615 trucks and 394 cars - Personalkraftwagen) plus 527 motorcycles and 201 trailers in I wave division and 578 cars & trucks + 415 motorcycles and 168 trailers in III wave division plus also between 4842 horses + 919 horse waggons in I wave division, 4854 horses and 823 horse waggons in II wave division and 6033 horses + 1529 horse waggons in III wave division.
Polish historian and participant of the 1939 Campaign (chef of staff of Army "Cracow") - lt. col. Wladyslaw Steblik - writes in his monograph of Army "Cracow", that one of the most important advantages of German infantry / Gebirgs divisions, was the fact that they were always able to form a motorized Kampfgruppe or to quickly transport some of their forces (let's say one or two battalions, for example) by trucks to a certain place, if only it was necessary. And I fully agree with him, because I know that he experienced this and that there were such cases during the campaign.
I already knew the horse, not the panzer or motor vehicle, was the German Army's biggest means of transport
Even if we assume that the horse was more important than these hundreds of trucks, cars and motorcycles mentioned above, we still have to admit that this situation didn't change until the end of the war. Even in 1943, 1944 and 1945 the importance of horses in the Wehrmacht was exactly the same or even bigger than during the Polish Campaign.
For example on 12.01.1945 Wehrmacht had got 1,136,318 horses (including 923,679 in active frontline service) and only 10% of all German divisions were mechanized while the remaining 90% relied on horse-drawn transport. And here it's worth reminding that in Polenfeldzug 25% of German divisions were fully mechanized, which is 2,5 times more.
In fact the later in the war, the worse was the motorization level. In 1939 level of motorization was one of the best.
It cuts through Poland, e.g. the advance on Warsaw, the dash across the Corridor, it races towards Brest, but by far the bulk of the fighting and effort is made by infantry/Gebirgs divisions
It rather fits to the Westfeldzug in 1940 and later to the campaign in the USSR in 1941, than to Poland 1939.
During the Westfeldzug Panzergruppe "Kleist" only cut through France / Low Countries to the English Channel, but the bulk of combats was fought by infantry. During Fall Rot it is even more visible - then armour played a negligible role.
There are words which carry the presage of defeat. Defence is such a word. What is the result of an even victorious defence? The next attempt of imposing it to that weaker, defender. The attacker, despite temporary setback, feels the master of situation.