"Fish" isn't a real type of animal, it's a term of convenience for similar looking/acting things that humans have lumped together.

Its taking that back to the medieval level of "whales are fish"... Which ignores that key difference of them breathing air and not having gills.

There isn't a simple evolutionary definition of "fish", not the same way there is for, say, mammals. If you found the common ancestor of everything we call a mammal and said "everything descended from this one is also a mammal", you'd be correct. If you did that for everything we call fish, every animal in the world would be a fish. Also, we decided which animals were fish mostly on vibes, so without a clear definition you can pedantically argue that everything is a fish including mammals.

Mammals are fish