or an alien wouldn't change that

And this isn't a baseless assertion? Well, there's two questions that pop in my mind when I look at a statement like that: Change for who? Us or them?

Of course for them, in that specific time when the Engineer walked the Earth, it wouldn't change anything. To them he would just be a God. Gods and aliens are interchangeable from the perspective of a primitive mind.

But we're not talking about them, we're talking about us, at least people from the onset of the Scientific Revolution to Contemporary, and how we would perceive a mythological document, such as the Bible (sadly historical to some folks), throughout time where the concept of aliens and space, general relativity and physics exist.

"Let's see.. Just Some Guy... One of the Prophets but not THE Prophet... oh shit, 15 FT TALL ALBINO MAN FROM SPACE." It presents problems to say the least..

I get it, you're a pretentious prick, you can stop replying now.

Ouch. Sorry you feel that way, man. Maybe don't emotionally invest in a fictional world of aliens and space marines if a simple discussion is gonna make you this salty? Idk.. good luck with that ;)