Evidence is what counts. No judge will believe in your revelations just because you say so.
You really don't know much about how the Polish legal system works, do you?
Jon would have nothing to fear in a criminal prosecution, as the burden of proof would be on the prosecutor to prove that an offence has been committed. Polish law works on the inquisitorial system, which means that the judge is obliged to find out the truth It would be nearly impossible for any judge to rule that a crime has been committed if many witnesses independent of each other step up to confirm what Jon said.
I'm almost certain Kaczyński wouldn't want the utter humiliation of having his sex life revealed in court. The international media would have a field day printing the transcripts, and his reputation would be in tatters by the end of it. That's exactly why no-one has ever taken President Wałęsa to court over it.
As for evidence, as has been said, it would be up to the prosecutor to prove that it's false.