Yes, for me the thing that stuck out in this letter was "have no culture". I've had that said to me too, and encountered it in passing in discussions a lot more often (with the recent addition of "white people have no culture but violence"). All the things people are saying elsewhere in the thread about how some heritage communities in the US are explicitly racist, how a lot of POC might have experienced Irish-American culture as hate or racism, how the friends might have just been trying to warn them before they fell too far into a rabbit hole - could be true.

But the phrase "[white people] have no culture" tends to be a very specific marker that the friends are the ones who are pretty far down a different rabbit hole. :(