Yes - main benefit of EU is regulatory stabilization and open market. Ironically also this was working also before joining EU (most of the adjustment happening as requirement to join EU and implemented before joining).
You could get the money but you had to get bureaucracy to be right and transparent to cut down on fraud, and that helped the rest of the govt to have less fraud.
Tight global integration is not a bad thing. Even if we took at face value your argument that a strong domestic market protected Poland in that case, you can't cherry pick the one instance in which lower-than-expected integration was beneficial without also considering all the other times in which it was harmful.