I agree that Poland's survival in 1920, stopping the advance of the internationalist Bolsheviks within borders slightly smaller than those of the former imperial Russia for a generation, was a considerable and important achievement.
However, I find it much harder to believe that if the Red Army had beaten the recently improvised patchwork army of Poland (a country that had then only existed for less than 2 years) in front of Warsaw in 1920 that the Red Army could have likely gone on to sweep across the rest of Europe.
As Davies notes in his book, the Soviets wre still fighhting the tail end of their own Civil War and could only muster thirty something divisions for the Poish campaign, a fifth or a sixth of the size of the army Russia managed to send against Gemany and her allies six years earlier in 1914, and smaller, often considerably smaller than the armies that Germany, Britain, France, Italy, Austria-Hungary and the United States had each managed to field only a couple of years earlier in 1918.
Yes, in 1920 Germany was still in some turmoil and had a millitant far left minority, and yes the Allied armies were being run down and mostly eager to go home following the end of the war. However, I find it hard to believe that a serious attempt by the Red army to sweep through Germany and Western and Central Europe, impose Communism and abolish the democracy and national independence that many countries had only just won or preserved, would not have prompted a considerable response from the above named countries, including almost certainly the prompt recall and rearming of many recently demobilised German units with the full if temprary blessing of even their former foes.