If lack of deception is the goal - then I find saying "he is really good at reading" as deceiving as "he is really bright".
Yes, he is bright (partial picture) - but the truth is he is gifted (full picture)
Yes he is good at reading (partial picture) but what is left unsaid is that the reason he reads so easily and at such a young age is giftedness (full picture)
Personally, I do not have any issues with saying bright or "good at xyz" but complete honestly is not my goal. Avoiding a scene sometimes is, lol.
So: The librarian says, "That book is probably too hard for you," and the child says, "I've read lots of books in this series," or "It looks interesting, I'd like to try it," or "I really like reading." None of those answers are incorrect, nor are they dishonest. Saying, "Well, I'm highly gifted" is not a more honest answer-- after all, lots of highly gifted three year olds don't read at all. It doesn't give the most appropriate or necessary information, and is therefore a *less* honest answer than "I really like reading" or a parent's interjection of, "I think that she'll be able to read that book without difficulty."
I guess that's part of the problem here, too. "I'm gifted" doesn't really answer a lot of questions. It doesn't mean that classes are always easy and it doesn't mean that books are always appropriate (even if the kid can read them). Outside of the context of educational placement, it probably doesn't mean all that much, and a meaningless answer is as dishonest as an outright lie in my opinion.