Boring."
Insulting/severe I would consider. Definitely a personal attack without even insultinghim first. THAT is the type of behavior one usually finds on those DU threads, case in point.
That is not an attack on you. It does not disparage YOU... it disparages your comments. Fair game.
He did not insult you personally. He did not call you names. He stated, somewhat colloquially that what you said was boring. That may or may not be a fair judgement call but It is not an insult.