The thing with exercise is that, well this is the argument the guy who wrote the book makes, if that you have to do so much to lose weight that it won't take you the whole way to losing weight. Basically each pound of fat in your body contains around 3,500 calories in energy. Only so much of the exercise you do burns fat as energy, so that for the average man eating a normal diet (which contains still quite a lot of carbs) you would have to run 40 miles a week just to lose one pound of fat. If you've cut carbs right down in your diet it will accelerate a little more than that, but probably to only around 1.5 pounds in a week.
Most people who do three gym sessions a week of an hour at a time, will probably only be losing around 0.1 pounds or less basically through physical exertion.
Most weeks I do at least 3-4 hours of pretty hardcore training, and I have never lost weight through exercise. It was only previously when I stopped drinking beer and stuff like that that I lost weight. Which is why people think they're losing weight through exercise when they do it, because when you often cut down on cakes and alcohol when you go on a health drive. If you are just going to the gym every week and not changing your diet, you don't lose weight.