I don't know if it's true, but I often think the only real solution for weight loss is eating less, and that exercise is more of a way to improve general fitness rather than losing weight.
A caloric deficit is the only way to lose weight. You can burn off the calories that you eat, if you have the time and the energy; but the most efficient way is not to eat those calories in the first place and then exercise to improve your strength, cardio-vascualar stamina and flexibility.
You're also not mentioning the other half of the equation: calorie input. If someone eats the same amount of food, and walks more, then they're burning more than before when they were just sitting on the couch. Of course what often happens that the body starts wanting more energy for the new activity level, so many people end up eating more--no net change.
It should also be noted that there's more to exercise than just weight loss.
