He doesn't say that exercise is useless. A lot of people have misconstrued a small part of his book and claim that is what he said. He says that the evidence points to the idea that it might increase the appetite, and that we might overcompensate especially if we're "intuitively" eating. For the super-morbidly obese, it might be better to focus on getting the weight down before focusing on exercise as the risks might outweigh the benefits, and nowhere does he claim that any evidence points to exercise as a bad thing. Rather, he says there are plenty of reasons to be active for health reasons and is very physically active himself.
Basically, he's not a nutritionist or dietitian and doesn't claim to be either. He's a science journalist with a degree in physics. He's compiled the evidence for and against the hypothesis that dietary fat causes obesity or ill health, whether saturated or not, and the role of dietary sugar (including complex carbohydrates) in obesity. He doesn't claim to be a diet guru, but for some reason he is painted as one by his detractors. And even if one or more of his conclusions he bases on the scientific evidence is wrong, it doesn't mean all his conclusions are wrong. Nor does he paint his conclusions as scientific fact, unlike many others who will talk in terms of "common knowledge".
Nutritional science is just the latest of the science debates that he's reported upon. He started out doing articles and books on things like the Nobel Prize and cold fusion. He got into writing about nutritional science when he investigated the evidence about dietary sodium intake warnings, moved onto dietary cholesterol, and then onto fat. His book is pretty interesting if you can tolerate science based texts.
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