The Treaty of Versailes only decided on fate of formerly-German held parts of Poland (Greater Poland, Upper Silesia, parts of Pomerania and East Prussia). As such, even Warsaw was not confirmed part of Poland by the Treaty of Versailles. And Soviets didn't occupy any formerly-German held parts. So terms of Treaty of Versailles are meaningless in this situation.

The british foreign secretary during ww1, George Curzon, argued repeatedly for the polish state to have basically the borders with russia that it currently does. So the legal version is that the soviet union by only occupying lands east of the curzon line weren't actually doing anything that meant the western allies had to declare war over.

(Both France and the UK had signed more general alliances with poland later which theoretically should have been triggered. But for example the Polish-British Common Defence Pact was specifically aimed at Germany not the soviets.)

The more pragmatic version is that germany was a direct threat and the soviet union wasn't. I think there was some talk on declaring war on the soviets anyway that was quickly dismissed after the fall of france but realistically, it was better to fight one enemy at a time. Churchill even gave a speach at the time saying that at least this meant the nazis couldn't push as far east as they would like to without having to fight the largest army in the world.