It wasn't happenstance, though. There is a specific reason as to why humanity's depictions of demons look the same as those aliens, and it's a critical plot point. (Can't say more or I'll spoil one of the best sci-fi books of all time for those who haven't read it).
Edit: it's also highly debatable whether the Overlords are in fact the good guys.
Edit: it's also highly debatable whether the Overlords are in fact the good guys.