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).
A lot of it also was behind a requirement to basically "fix your shit".
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.
Much of the stabilization was due to the strong domestic market. Recall that Poland was the only country to avoid the 2008/2009 recession. It is tight global integration that causes recessions to spread.
Brazil also famously avoided the 2008-09 recession to a great extent, to name one example.
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.
Poland's growth does well when everyone is in the dip. Even in 2020 crisis Poland dipped less than other. Although the difference was less that time. 8 years of populist rule did harm Poland a bit.
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.