Sorry but I think you're mistaken here. Your example of someone out of shape killing the test doesn't really apply. I'm almost certain that were that out of shape person to exercise on a daily basis, they would be able to better apply themselves in every part of their life, including mentally. Exercise does help many mentally as well as physically. It's all related, your body utilizes energy better, your mood will be better, you will be more energized, and for those who have trouble focusing, it will even help you focus because you are more mentally alert. Granted you're not anymore "smarter", but you're better able to use the intelligence you do have. To say exercise plays NO role in MCAT success I think is a mistake. But obviously one day of exercise the day of the MCAT is useless if you're not used to working out..
If you're already able to focus and have plenty of energy then exercising probably won't help that much. But if you're someone who is low on energy, has trouble focusing... putting 30min-1hr aside for exercising can definitely help