As I said on the other Q’s about this. The Catholics have no meat Friday, but they can eat fish, so they think of fish as not meat. Even some non-Catholics think that way. I use meat to mean anything that is the flesh of an animal, it doesn’t have to be a mammal.
People who say fish is fish, not meat, well what about Alligator meat? I think that is a well accepted term. People also say turkey meat, but then some people consider turkey to be poultry or fowl.
In supermarkets often the meat department contains, lamb, beef, poultry, and pork. Seafood often has a separate counter or section. So what? The dairy department usually contains eggs. I don’t consider eggs to be dairy.
I don’t think it is wrong to define meat as only mammals, I also don’t think it is wrong to consider all animals, including those living in bodies of water, meat. They are both right. They are used differently by different people in different regions of the US, you just need to know to clarify if you aren’t sure what someone is trying to say.
In Spanish I am pretty sure they use carne to mean only mammal meat, and that translates as meat, so people who speak that as a first language, might use meat in English that way.