ITAR allows greencard holders, so not everyone's an American citizen.

Not only greencard holders but ITAR even has exemptions for non-US persons, that is those who aren't US citizens or Permanent Residents.

I don't work directly with anyone like this but I know SpaceX has a few, not sure of the exact number, employees who are neither citizens nor permanent residents.

From what I've heard the process of getting a non-US person ITAR cleared is extremely costly, both in time and money and is thus used very sparingly.

Yeah, I love Elon's American accent.

His maternal grandfather is American and he took the citizenship oath in 2002 -- Which is why America is pretty cool, many millions of Americans have foreign accents!

I'm pretty sure Elon considers himself thoroughly American at this point.

There are quite a few foreign born American citizens working at SpaceX, myself included. You may find this shocking, but seeing as English wasn't our first language, some of us even have foreign accents.

I don't consider them, or myself, any less American.

You jest, but being an immigrant is a very American thing. Visit it sometime if you had not, it's quite a different place from what many people imagine from media dramatizations.