Fish is meat. It's just that people are used to associating "meat" with "red meat". But if you press people, they are not going to say that in no way is fish flesh considered meat. They just don't generally use the same word to describe both things.

You don't need to know any details of taxonomy as a discipline to know that fish are animals.

For the record, the original meaning of "meat," was just food, in general. So we have sweetmeats, mincemeat, etc.

The exercise you're talking about, is an exercise in taxonomy. (Though I agree it doesn't require special expertise to engage in taxonomy.)

It only requires a working brain to look at two similar things and categorize them similarly.