I agree with General Battuta. Any sufficiently advanced technology can easily be mistaken for magic.

They might be godlike to us, but who's to say they don't have the ability to read Terran computer files (or, as Battuta suggested, Sam's brain) and discern from it an appropriate name? Perhaps they identified with the Hindu god Vishnu, much as Khonsu II identified with ancient Egypt, and adopted the name for use during communication with Terrans. It's also equally possible that Sam thought of it, and it just stuck. It
did almost seem like he coined it himself, as the good general mentioned.
Battuta also has a point about the communication between the Preserver and the Dante. I don't recall the conversation ever being acknowledged by Sam or anyone else, so it's reasonable to assume it wasn't in English. Darius probably didn't want to break the fourth wall by outright saying that they were communicating in an alien tongue, if he had considered it at all.
