That's just a signature though, and the implementation could do so many things for complex and/or strange data structures.

So in a sense yes it's "just" this function that could do anything following the signature.

But now you're in the same place you already were, looking at a simple type signature with massive hidden implications, and if you treat it like a list you'll miss those implications.

"flatmap generalized to a vast field of dissimilar types" is much more complicated than just "flatmap"

Yeah, in formal terminology that’s “join”. Join + return + monad laws is a perfectly reasonable definition of a monad.