An otherwise almost imperceptible decline in mental function could have have a huge impact on ranking among top chess players simply because at the top levels the margins between players are so small.

If chess Grandmasters dropped in mental acuity as fast as those Luminosity results suggested, a 60 year old Grandmaster could be beaten by any young novice.

Also worth mentioning is that competitive chess is favors very quick thinking and action in a way that most careers don't.