Can diet cure all of hair loss? Hell no. But there's definitely a role it can play in various forms of male balding.
There is of course another point that must be made here. Most people who are healthy are not healthy because they're athletic. They're athletic because they're healthy. While forcing yourself to go to the gym can cause a bit of an improvement in your overall health if you're in the later stages of cancer you're probably not going to be feeling too spry no matter how many times you go lift weights.
Meanwhile if your body is in proper operating condition then your energy levels will be higher, your hormones will be in their proper ratios, your outlook will be better and you'll have more motivation. All of these translate into leading you to have more of a want to be active. A few months ago I finally sorted out all of my health sh*t (basically started doing the opposite of what the USDA and NIH recommends) and since then my attitude has become annoyingly positive, and I NEED to exercise and be active. Hell I can't go out for a walk now without feeling a severe itch to break into sprints.
The healthy internal state often times actually drives healthy behavior so that twin with a fuller head of hair might just be the athletic one because he's got less internal sh*t screwing with the various parts of his body (like his hair).
Edit: that being said, chicks dig a muscly bald guy over a scrawny bald guy with gyno, hands down. Getting fit > worrying about your hairline.