That is an absolute nonsense. I don’t know where you get your information. Yes diet might help, but exercising regularly, not even exercising, but simply walking couple-three miles a day would help a lot.
The more you exercise is the better, but even 3-4 times a week is a huge plus. Most of our weight problem comes from being inactive. When I did not have car and had to walk a lot I was size 30 now, well it doesn’t matter what size I am now, but I am not 30 any more. And I eat much less today compare to what I used to eat then.
Unfortunately most people think this way, and it's no wonder so many are so fat and overweight, and it's far from being that simple. Yes, any activity at all that you do is a benefit, but what you eat plays a great role on how you look, and you cannot pick an area of the body and train it to lose fat there.
What you weigh, and how fat you are, are 2 different things. You do not have to be overweight to have a gut. I don't think the poster said he was overweight either. How your body looks comes down mostly to body fat composition, not what you weigh. You can have multiple people with the same height and weight and look entirely different. It all depends on what your body fat percentage is. The higher percentage of your weight that is fat, versus lean muscle, the fatter you will be and look. I am sure you have seen people who look very thin and who do not weigh much, yet they may have a gut. They have too much body fat. Regardless of their weight.
So if somehow has a gut, what they weigh doesnt matter, they have excess body fat. And to get rid of that fat, a combination of proper nutrition/diet, cardio, and strength training is the only guaranteed way for fat to leave the body, while mainting lean muscle mass. The more lean muscle mass, the more fat the body burns, the better you look, the faster your metabolism. Also, age doesn't matter one bit either.