Communist parties are technically legal, as both communism and communist symbols are explicitly ruled legal, but it’s court precedents which are of lower rank than state law.

But propaganda for revolution (overthrowing of state system) and totalitarism (legally unspecified what is totalitarism) are illegal thus in practice every communist org is constantly dragged into courts over and over and over again and legally harrassed in other ways till they give up. As you can imagine any organising is pretty hard under this, we have one communist party left and it basically ceased any activity.

And isn’t Poland filled to the brim with reactionary anticommunists?

Yes. Plus there is very striking unity in all media about this, the information and propaganda bubble is real, is constant and uniform.