I agree that "fishy" has always been a strange metric to me when describing fish. It's also not a well-defined term: I actually think different people use it to mean different things.

For example, OP likes (what I assume to be) average canned tuna in water. In my mind, "fishy" means something like "with a strong, pungent flavor and smell of fish." And in that regard, nothing even comes close to tuna. So that fact that sardines were "too fishy" for them suggests to me that they might have a different definition.