I don't think it has to do so much with reflexes, in any sport. I've been reading about how a batter, in baseball, from experience, learns to predict the pitch at the top of the pitcher's delivery. This happens in tennis and other sports, and it happens in boxing.
I don't think that the ability to run faster or jump higher helps in boxing (other than in dire emergencies)...Boxing is a pretty specialized thing in that it happens under fire; you get hurt right now if you are wrong.That is a pretty special condition that affects a lot of individuals and their athleticism.