As far as I understand, there isn't really a consensus on what "natural-born citizen" actually means in the context of the US constitution. I'm sure some huge team of lawyers could argue that Musk somehow is "natural-born citizen".
There are some edge cases like “your parents are american but you were born on vacation in france”, but this one is about as crystal clear as you can get: elon was born in SA to a mother who was born in CA to canadian parents and a father who was born in SA to south african parents. He is a naturalized citizen of the US, and ineligible to stand for election for the presidency.
Yeah, which sounds like a fair guess, but the only way to know for sure is for it to be tested in courts, otherwise we're just guessing :) And as we probably all know, having large pockets for lawyers and lobbyists, makes a lot of what we think isn't possible, possible.
... other than "person x has never been and is not presently a citizen" this is as clear as it can get. Naturalized citizens are not eligible to be president, there is no gray space on this matter.