The Soviet Union and Japan were fighting a real border war in 1939 while Europe moved toward war. Soviet timing in Poland was affected by the need to finish that eastern conflict first. The divisions later remembered as elite Siberian formations had already trained carefully and gained combat experience against Japan. Their reputation therefore came from prior success and preparation rather than simply from arriving fresh in the west.
Notable Point · 2:53:45-2:54:47
The 1939 Soviet-Japanese war hardened Siberian divisions
A land-war aside connects the Soviet victory over Japan in 1939 with the combat reputation of formations later transferred west.