They must have done something to provoke the attack, I've walked around Poznan in some terrible states over the past few years and never had any real issues. I can believe it about the police ignoring it as the police here are a joke but there must be more to the story than you are letting on.
From what I've heard, Spanish guys are coming to Poland in droves, trying to get laid. Somehow, it wouldn't surprise me if these guys actually went to a brothel and were beaten by pimps for not paying (or worse, for treating the girls badly).
Spanish men are hormone filled macho men at the best of time - it wouldn't surprise me if one of them actually tried pulling a knife on someone and then got his arse well and truly kicked for it. There's countless incidents in Spain where Brits have got involved in fights with locals, only for the Brit to end up stabbed by a cowardly Spaniard.
Let's be realistic - no Spainsh guy is ever going to admit that he got a beating for doing something wrong. Spanish guys, by nature, are even worse than Polish men when it comes to not accepting that they can be wrong in any way.