> I've heard a self-proclaimed vegetarian state that fish are not meat. I guess he was brought up old style Christian.
You are likely correct. At least in some Catholic countries, when they talk about "meat", they exclude fish, apparently because that is permissible on days when other meat is not. Notably, in Spain a "vegetable sandwich" is a sandwich with tuna, eggs, mayo, a slice of lettuce and if you're lucky a bit of tomato.
I do not know if there is any historical relation to Jewish dietary requirements, where the "meat" food group does not include fish, which is considered "neutral"¹, same as vegetables.
¹ Assuming you consider said fish to be kosher in the first place, which not all fish is.