Instead of backing military force against Nazi Germany's call for the dismemberment and annexation of Czechoslovakia, Poland worked against it and use the opportunity to vulture off some scraps from Czechoslovakia as part of their revanchist goals dating back to border wars in 1920.
As far as counterfactuals go, Munich was arguably the last -and best- chance to stop Nazi Germany. Poland not throwing Czechoslovakia under the bus and permitting Soviet troop transit to defend Czechoslovakia was how Nazi Germany could be stopped. Instead, Poland showed itself unreliable to both its attempted Central European alliance system and to France, Poland's traditional Great Power backer. Poland collaborating with Nazi Germany in the dismemberment of Czechoslovakia convinced the Soviet Union that Poland was at best a Nazi patsy and couldn't be trusted. (Hitler had spent much of the 1930s trying to get on good terms with Poland and enlist them in his crusade against Judeo-Bolshevism.)
Czechoslovakia also possessed the largest arms factory in Europe (Skoda Works) and substantial reserves of the iron, steel, and rare earths that Nazi Germany desperately needed. I'll have to break out The Wages of Destruction again, but I'm pretty sure a good proportion of the war material used in the invasion of Poland and France came from either Czechoslovak factories or their material stocks.
Yes, Poland did get involved in picking up some Czechoslovakian lands, but the rest is assigning blame to someone who had barely any responsibility, while letting those of the hook who actually could have done something about it.
At that point in time, Germany had a treaty of friendship with Poland, and had made no move against it. The Soviet Union, on the other hand, had fought a war against Poland and was very far away from acting friendly. Neither side was trustworthy, but at that point the Soviets had been the more recent threat.
Pretending that it was oh so evil of the Polish not to let the Soviets walk through their lands is just absurd. No one trusted the Soviets, and for very good reason. They weren't some benevolent power trying to protect Czechoslovakia out of the kindness of their hearts. Their interest lay in gaining control over as many puppets as they could, as evidenced by everything they did afterwards. If Soviet troops had been allowed to enter Poland, there is no way Poland could have defended itself against any Soviet attempts to get involved in internal Polish politics. The Soviet declaration to "defend Czechoslovakia" is as meaningful as Germany "protecting Danish and Norwegian neutrality" by invading. You are basically saying that Poland should have taken the risk of throwing itself under the bus to maybe protect someone who Britain and France didn't want to protect either.
Poland had very little influence on the outcome of the Sudetencrisis. You want to blame someone, blame
a) Hitler, for pushing the whole thing
b) the Entente for being giant hypocrits who didn't want to let German speaking people have the right to self-determination after WW1, that the Entente claimed was oh so important to them
c) France, for chickening out of the Alliance with Czecoslovakia due to not wanting to fight a war just because Germany wanted to have land mostly settled by Germans
d) the British military for being ill-informed about the strength of the German military and telling Chamberlain that they wouldn't be able to send any noteworthy amount of troops to the continent, which, alongside the distaste of the public for another war, caused him to go for all out appeasement
e) and following up on that, the refusal of Britain and France to stand up to Germany even though German military commanders virtually begged them to do so, as they would have attempted a coup if Hitler had declared war, so long as Britain and France just kept up the pressure in Germany
Then, and only then, can you even think about including Poland on that list. And even then, expecting them to let a foreign army of a questionable if not outright hostile regime "move through their lands" is a very odd stance to take. No one lets armies of a non-allied nation move through their territory. And you sure as heck don't let a hostile regime do that, regardless of how good it claims its intentions are.
It is the height of ignorance to claim that Poland's decision somehow forced the Allies into appeasement. It did no such thing.