You don't need to think P.D. Mangan's diet is healthy to notice the obvious fact that Bill Gates looks old. There's clear selection bias - he thinks his advice is so great because it appears to work for him (he probably doesn't have insomnia, hypertension etc.), and he chose Bill Gates because he appears to be in a bad condition. There's some chance he'll die e.g. of a heart attack before Bill Gates, but it's clearly not over 50%.