The original comment was clearly a joke, because this is a question to which nobody has the answer.

i'm sorry what? the OP was clearly confused by the original commentor as evident by them asking who mrs norris actually is. and nobody could possibly see "mrs norris is the old divination teacher" as humorous be honest now. mildly amusing and an interesting theory sure. but a joke? cmon.

This is essentially funny because it's absurd; nobody would possibly believe it was a serious theory, because it's against McGonagall's character (as per the Malfoy ferret incident)

mrs norris being the previous divination teacher isn't out of the realm of possibility at all. and he made no mention of it being a theory or anything unlike other commentors.

and frankly overreacted pretty significantly.

how so? i didn't insult him at all til he called me a troll. i first asked him to not post fanfic plot points as fact (something that happens quite a lot in this subreddit) he then said he didn't read fanfic (why he didn't just say it was a joke then i have no idea) so i asked where he got his information from. what's an overreaction about that?