My experience differs. Just had a game with Germany, declared war on Italy very early, this resulted in Gb declaring war on me. After I invaded Gb from Scapa Flow, the Soviet Union declared war right there even with Poland in between. I did not take London, but it was a very weird game full of foo bar. In August 1940 there were no Allies or Comintern left. But it got better. Finally I could enact the Vienna Accord and all in a sudden, Poland came back into existence, because they got one province from CZ. And with them the Allies.
What I want to say with this example, the game is full of triggers and eventually the Soviets declare war even if you share no border at all. You can even reanimate dead fractions.