Jumping recklessly in at the middle: even granting your premises regarding the scope of 'ought', it is not wholly clear that an alien "ought" is impossible. As timtyler pointed out, the Babyeaters in "Three Worlds Collide" probably had a would-want structure within the "ought" cluster in thingspace, and systems of behaviors have been observed in some nonhuman animals which resemble human morality.

I'm not saying it's likely, though, so this probably constitutes nitpicking.