The bottom line is you will need regular exercise no matter what.
Diet is still very important. If a person is exercising regularly, they might be able to eat more “junk” food (I’m including most carbohydrate-rich foods in this category due to their role in gaining weight, not just processed food and sugar) without gaining weight. Depending on what they are eating, however, it is still possible to exercise often and gain weight as fat rather than muscle.
If someone doesn’t exercise and eats the same foods, they will gain weight. But if they eat the right foods, that sedentary person would be able to get away with only a small amount of exercise (say, a walk around the block every day?) without gaining weight.
The thing this common belief ignores is that metabolism is related to muscle mass. The more muscle you have relative to your total weight, the higher your metabolism (of course, other things factor in, but muscle mass is directly related to metabolism). Guess how you build muscle? Excercise/working out.