I had this exact same argument with someone when he said he was a vegetarian. We were looking for a restaurant to eat with coworkers. Two sentences later he said he could eat fish, so I said “fish is meat, I thought you were vegetarian” expecting something about “it’s just easier to say I am, but really I’m a pescatarian”. But no. He said “fish is not meat”

I think people get “animals” confused with “mammals”. Also there’s some weird thing with Catholicism where fish isn’t defined as meat for the purposes of Lent. But man it’s infuriating to meet someone like that.