To people who downvote because this is "useless": This question stemmed from actual conflict which arose on a child SE site between several users.
To people who downvote because they assume the purpose of the question is allowing the OP (me) to engage in such behavior: opposite. I was the one who called it out. But I didn't have a meta policy to back up my opinion that "this is bad", thus I created this question.
User 1: My post XYZ is stupid
User 2: Your post XYZ is stupid.
Now, in isolation, the second statement is a clear violation of "Be nice" policy:
Name-calling. Focus on the post, not the person. That includes terms that feel personal even when they're applied to posts (like "lazy", "ignorant", or "whiny").
Does the fact that the second statement merely quoted the very same thing the target of the statement said about him/her self (or their own post) earlier, make that second statement less of "not nice"?
P.S. I also asked a largely independent but related question of whether statement from User 1 is considered "not nice" in the first place, despite being self-directed