...and sell nothing to anybody.
LOL. We don’t need to sell it to anybody. It’s only for our own purposes, just for the energetic independence if you know what I mean. Anyway,...
Poland has an official unemployment rate of just under 10%. But a huge part of why the unemployment rate is so "low" is because Poland has been able to export surplus population elsewhere in Europe under EU freedom of labor rules. If 2 million Polish citizens have to come back home because they lose work permits, what do you think that would do to Poland's economy? Easy: it would collapse.
Second, you seem to think Poland doesn't need to export anything to anybody. Except Poland's economy would collapse if it can't export agricultural products, automotive products, pharmaceuticals, etc. Poland's economy is hugely dependent on foreign trade, and the vast majority of Poland's trade is with the European Union.
So yeah, they could quit the EU and the country would collapse. But hey, coal!