It is sort of justified, though Poland's actions are also absolutely justified.

Here you can read the detailed explanation of USSR's negotiations with Britain and France that ended with Molotov-Ribbentrop pact.

The key point about Poland is that they could have allowed the soviet army to pass through to go help the Czechoslovakia, but if they did that then the soviet army would probably have subjugated Poland on the way back. But not letting the soviet army pass through resulted in USSR making a pact with Germany.

Not only did Poland not help the Czechs. They actively helped the Nazis. They co-invaded Czechoslovakia and annexed a handful of border towns (some of which the Czechs had annexed while Poland was at war with the Soviets in the 1920s). Rydz-Śmigły even negotiated with the Germans to make sure that the Hungarians also got pieces of Czechoslovakia, as he wanted a shared border with Hungary. This helped legitimize the Nazis' invasion. It also undercut Czech and French attempts to stop the Nazis with a unified front.

Poland was in the interwar years nationalistic, expansionistic, militaristic, and bellicose. It had border disputes with Lithuania, the USSR, Czechoslovakia, and Germany. And they were willing to use force to acquire and keep land. They fought a war with the Soviets to gain parts of Belorussia and Ukraine. And they initially tried to take all of Belorussia and Ukraine. They invaded Czechoslovakia. They sailed warships into Danzig to enforce what they viewed as their treaty rights. They even earlier in the 30s threatened to invade Germany itself (this was before Hitler's expansion of the Heer).

Poland was a perpetrator before it turned into a victim. The Russian view is a biased one. But it is one with a kernel of truth. Their use of (sometimes literal) gunboat diplomacy helped legitimize the use of force. Their unwillingness to work with other countries helped make the French attempt at containing Germany ineffectual. Their active collaboration with the Nazis helped destroy one of their greatest potential allies. Piłsudski's and Rydz-Śmigły's policies didn't cause to WWII. But in retrospect, Hitler continually used Polish policies to his advantage.

The Poles didn't (rightfully) want the Soviets in their territory. That didn't justify Stalin allying with Nazi Germany to invade Poland together.