I agree this is the most likely reason, however to me this indicates a limitation in the guarantee/alliance system that, if addressed, would allow for more nuanced and historically accurate gameplay. And that it wasn't addressed, even though the devs were aware from the start of its existence (as it's not their first HOI game) is disappointing.They don't have the Soviet Union invade, because then the Soviet Union would be stuck in a war with the Allies (Poland is part of the Allies, after all). This outcome is far worse for the "historical" path than letting the Soviet Union grab land after Poland surrenders.
(a very basic solution would be enhancement to current alliance system -- a (scriptable) switch controlling whether a member of alliance who refuses to join a war against another member of that alliance... gets ejected from the alliance as result, or not. That way you could easily have what actually happened -- Great Britain and France refusing to join the war against Soviet Union, but still remaining members of the Allies, while the war in the East remained limited to Poland/Soviet Union, and could be resolved as such)
For what it's worth if I recall the previous streams right the territory of Poland gets divided between Germany and Soviet Union *after* Poland loses the war with Germany, by simple event that fires and flicks who is the new owner of eastern provinces. But that's far from satisfactory.
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