Right, the first argument to useMemo must be a function, but the first argument to useCallback need not. That's what you meant, right?
useMemo returns a value which is the result of calling the function it is given once the dependencies change.
So useCallback is basically useMemo called with a function that returns a function.
It returns something with the same type you gave it. If you give it a function it gives you a function back.
In typescript it's declared to take a function but in practice it doesn't check, and can be used to stabilize the identity of anything.
useMemo otoh _does_ only take functions.