I agree that there are cases where claiming high IQ is an exception to some social rules, but I think that a lot of the differences between that and the other self-claims you use as examples here come down to a social norm against unsolicited boasting.

"The main thing I'm good at is art" isn't a boast, because it makes no claim about how good the individual is at art relative to other individuals, or about the relative value of art to other talents. You wouldn't assume this person is claiming superiority to someone else who would say that "my main talent is singing," for instance.

A more comparable statement would be introducing oneself with the claim "I'm Joe, and I can bench press 500 pounds."