I got confused about this too, when I found out that traditionally meat has referred to warm-blooded animals. But that’s not universal across cultures, “warm-blooded” has been an obsolete term for quite some time in biology, and thermoregulation is kind of a silly condition for something being considered vegetarian or not. A fish is clearly not a vegetable, unless in an induced coma, and I doubt they person only eats comatose fish.