If John Carmack or Bill Gates thought of themselves as geniuses, they wouldn't have accomplished a fraction of what they did. Nothing is more destructive to someone's intelligence or creativity than believing the hype that people put on you, and that goes 10x if that hype comes from yourself.

Put another way, I consider both of those guys geniuses, but if either of them made that claim about themselves, I would have to reconsider the position.

Put another another way, no good can possibly ever come from acknowledging your own genius, even if that was something a person was capable of perceiving about themselves, which I don't think it is.