As others have said, you’re seemingly ignoring the existence of EU regulations which apply automatically without needing to be transposed into local law, and of the Court of Justice of the EU whose decisions are binding for local courts.

Both of these were established by the treaties that were in place when Poland joined, and further reaffirmed in the Treaty of Lisbon (which was supposed to be the clarified and rewritten European Constitution, but that was a bridge too far for some countries’ voters, so instead it became a web of amendments over the existing treaties).