Look up those tests and see what they selected for, its not as much as raw physical fitness but rather how their bodies reacted to stress + a host of other pyschological factors + flight training. Yes, it is without a fact that they are no slobs but calling them the most fittest is also hyperbole and paints this image of hyper fit astronauts which wasn't true back then. They also didn't care much about long term effects of space travel on the body back then because missions were very short back then.