Bobby Fischer, and a chess playing computer, highlight the difference between rationality and talent. Talent is simply the ability to do a particular task well. I tend to think of rationality as the ability to successfully apply one's talents to achieving one's reasonably complex goals. ("Reasonably complex" so the computer doesn't score very high on rationality for achieving it's one goal of winning chess games.)

Someone with limited talent could still be rational if he was making the best use of what strengths he did have. In a very real sense, we are all in that situation. It's easy to imagine possessing particular talents that would make achieving our goals much more likely.

That said, certain talents will be correlated with rationality and it's an interesting question to see to what extent chess is one of those talents.