His involvement with the game has been confirmed, but that only deepens the mystery.
Bungie is a giant name in gaming. They created 'Halo' after all, which is one of the biggest and most popular franchises there is. Paul McCartney was a founding member of the Beatles, which is the biggest and most influential band in the history of music. Now the two are working together.
McCartney confirmed that he's working with Bungie on their new project. He's writing music, presumably in collaboration with company composer Mary O'Donnell. Of course, as is often the case with Bungie, every new piece of information just leads to more questions. Questions like "what kind of game is this anyway," "what is Paul McCartney doing working in videogames," and of course "will Maybe I'm Amazed play during the end credits? Because that would be awesome."