It's more that, as I understand it, "fish" isn't a coherent phylogenetic category so much as a convention-based descriptive grouping of certain characteristics. I don't think of whales as fish, no. But an exclusion based on eg tail fin orientation or lack of gills is based on convention rather than strict taxonomic practices. So if someone wants to weigh the characteristics differently and include whales in the term fish I would at least hear them out.
As the saying goes, there’s no such thing as a fish - that is, there is no reasonable definition that both strongly selects for fish was, while rejecting non-fish.
I know it’s not the point, but how does a flounder get categorised in this system? Are they vertical or horizontal?
When they swim it usually has their tail horizontal.
Different category but well explained here https://eukaryotewritesblog.com/2021/05/02/theres-no-such-th...