No they signed a non-aggression pact which everyone else had done at the time. The pact had spheres of influence, a clear "don't fuck with us when we do things here and we won't fuck with you do things there" there was nothing in it that said anything about some military alliance to invafe Poland.

Molotov-Ribbentrop was signed 2 weeks before the nazi invasion of Poland and 1 week before it was signed by the supreme soviet. You don't attack a country together within a week that requires months of time planning.

The Nazis invaded Poland and the polish government went into exile so the soviets rushed in to prevent land that was historically part of their territory and inhabited by many ethnic Belorussians and Ukrainians from being occupied by the Nazis. Land that had been taken from them by Poland in their war of aggression to achieve their maximum historical borders even though as I mentioned had spent so much time outside of Polish control now had sizable Belarusian and Ukrainian inhabitants.

Of course, were we to live in a timeline where the soviets did leave all of Poland to be occupied by the Nazis. Idiots like you would be going on about how stupid and evil the soviets were for not doing anything from preventing the land from falling into Nazi hands. Doing nothing to protect the people living there when they had the opportunity and allowing the Nazis to get a border even closer.