well, Poland didn’t have restrictions on private firearms ownership (as far as I can read). But the fact is that they were a flat land with no rivers or mountains to defend them from a two-way attack (both germany and russia attacking at the same time)
Conceivably, had the Russians not moved in, the Poles could have held out long enough for Britain and France to come to their aid (of course that aid was never going to come), but at least it would have given them a fighting chance.
A lot of people don’t realize, that the Germans lost 15.000 soldiers during the September campaign.