While Paul Bettany had actually been part of the Marvel Cinematic Universe since the beginning, as the voice of Jarvis, he only became a tangible character in Avengers: Age of Ultron and may have only really begun to pay attention to the larger MCU at that point.
As somebody who wasn’t super into comic books as a kid, he knew the names of the heroes, but not necessarily the names of the people who wore the costumes.
It probably seems almost funny now, as even people who didn’t grow up with comic book characters like Iron Man or Captain America all now know who Tony Stark and Steve Rogers are, but there was certainly a time where that wasn’t the case.