Fake news
This is the fake news, and liberal propaganda. Far-right nationalist is marginal groups in Poland. Most of them are normal people who want to commemorate Indepedence Day. Please pay attention to the crediblity of the sources. Believe finally, that in Poland there is no dictatorship or anything like that. I am a normal Pole, and I do not see that there are fascists, nationalists or Nazis in my country.
We do pay attention to credibility of sources, in fact we put massive emphasis on it; if you think those aren't credible, likely you're trapped in one or another sort of information bubble.
Nobody is saying that they aren't a marginal group. The fact is, the march did happen. Otherwise, how do you explain why your own president condemned the march? If it was fake news, he surely would have condemned the press instead. It's pretty much the same as the events in Charlottesville, Virgina. A marginal group succeeded in getting a lot of attention by holding an event.
My main concern was that this march is a tradition, and it has been organized since pre-war times. It is not intended to demonstrate support for fascists. Simple people participate in it.