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Yes he does. If by "Which functions nat -> nat are computable" he meant "functions written in some typed language" then the answer is vacuously "all of them".

Not all functions of type nat -> nat are computable in any formalism. You can write plenty of programs that are really of type nat -> nat in say, Haskell, that try to compute a non-computable natural function. They just can't do it and they'll never terminate for some inputs. But they're still perfectly well-formed.

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