Before WW2, Poland was pretty antisemitic. In the 1930s, for instance, youth nationalist movements had advocated for "ghetto benches", so Jews couldn't sit with Poles in university classrooms, following growing violence against Jewish university students. Which given the rising tide of antisemitism in Europe, wasn't exactly shocking, but antisemitism did certainly exist in Polish society. It's certainly different than American university antisemitism of the era, which was limited to quotas (which were sometimes only subtly enforced)--Poland didn't have university quotas at all for several years after WW1, but they returned.
During the war, a great deal of the Holocaust was perpetrated and assisted by local collaborators. However, this was much more common in other countries, like Ukraine and Lithuania, than in Poland.
And after the war, there were incidents of antisemitic violence among Poles. The most famous is the Kielce Pogrom. These effectively ensured that survivors would not return, barring any chance at a revival of Jewish life in Eastern Europe post-war.
On this subject, I would highly recommend Antony Polonsky's My Brother's Keeper: Recent Polish Debates on the Holocaust, which is a discussion of essays/articles/etc which were written in the 80s when a flurry of debates/discussions on this subject in Poland occurred.
On the accusation of the Polish commenter being racist against Russians, that seems like an exaggeration from the banned user.
Reading the whole thread again, there doesn't seem to be much "harassment" as stated in the modlog, as much as a heated debate between the two parties.
From a lemmy.ml user perspective (which I'm not), I could see why they would complain about a political bias against the USSR.
One potential improvement point might be for mods to add historical sources to why they consider a comment misinformation. Also, banning this type of users and removing those comments (and leaving the others) might lead to an echo chamber effect on lemmy.world (mirroring the one on lemmy.ml).