As Japan if I were to attack Russia, I’d go through China to get there. Germany should be able to keep the USSR too busy to spend money on several fighters/Tacs to send east if you attack Siberia.
If Japan doesn’t attack the Russians they still will need to defend Manchuria from the stacks of Russian soldiers who start in the area and can attack after turn 4, so you’d still have 30+ IPCs devoted to the area with no gains.