Doesn't seem that odd to me. Lots of correlation/causation there, and there's truth to it - smart people do well and learn faster, one makes genuine progress (in the game), one learns heuristics which one will try to apply elsewhere (of questionable value; Tetris effect), early on the mental exertion may be helpful and one can imagine that some kids don't care enough about 'regular' intellectual activities but will about chess, then you have the usual biases and Hawthorn effects.