You can use functions without knowing their inputs and outputs, and your functions don't need environments that somehow allow them to lookup the other functions they're going to call?

Messages and function calls aren't quite isomorphic, but they're pretty close. (They get really close if you include coroutines and coroutine-like things, to cover the non-1-to-1 relationship between messages and replies that messages can have.) I'm not sure the difference can sustain the implication you're trying to draw. Especially if you include RPC in the function world.