The man on the moon is likely the "face" of the moon (if you try, you can kinda see it). And since the moon has a really good point of view from way up there, if he can't see it, no one can.

i.e. the man on the moon can see all, but not [that]

This is my understanding.

'The man on the moon couldn't see that shit' means the exact opposite of 'Good enough for government work'.

I once worked with a Dutch guy who would use the word "dinggus" (pronounced sort of like dingus, but without the emphasis on the g) in place of any English noun he didn't know. Took me a couple of days to work out that it was a placeholder word and that it could refer to something different every time he used it rather than being a name for a specific thing