This is a reference to carbohydrate metabolism. Your body burns of more carbs as you eat them whereas eating more fat leads to more fat storage. Ultimately, you're going to gain body fat if you overeat calories, it's just likely not coming from carbohydrate calories unless you are eating an insane amount of them. From a practical standpoint, most people who overeat calories aren't eating 100% of their calories from carbohydrate and/or eating limited fat calories. It's typically going to be a diet comprised of both high fat and high carb, with most of the fat serving as the substrate for fat storage.