Lots of cultures don't class fish in with most meats. In traditional Catholicism the rule against meat on Fridays didn't apply to fish and likewise with Japanese Buddhist rules against meat eating. It's sort of like how most people consider tomatoes to be vegetables even though they aren't, scientifically.

Yeah, in 21st century English fish is a meat but it's non-centrally meat so the meaning is clear when you apply Gricean norms.