According to the dictionary, ‘is’ and ‘was’ are respectively a present and past tense singular of the world ‘be’. The definition of the word ‘be’ (or at least, the one I find relevant here), is “having the state, quality, identity, nature, role, etc., specified”. This seems like a significantly more important part of the sentence to be emphasizing here.

On the moon he is the alien.

As opposed to who? I still just can’t understand why you would emphasize ‘he’, when there are no other subjects that we could be talking about. Like, obviously we’re talking about Neil, so why would you put emphasis on it? On the other hand, the fact that he actually ‘is/was’ an alien is pretty surprising to think about and thus the important bit.