He didn't actually say the McGuffin comes from any of the earlier films, even if it almost seems to imply it by saying it's 'connected'. Maybe it's too 'connected' to Spielberg and Lucas themselves, or to the public consciousness, or something else like that. He also mentions removing 'the offending parts' of it, which is interesting. People tend to get offended over religion the most, so maybe that's the biggest clue that it's another religious artifact or something to do with religion.

By the way, one of the most famous McGuffins was the actual Maltese Falcon. When it came down to it, the object itself wasn't the real point of the film, but it did serve as the thing that moved the story along. And that's the genius of it.

--SKot