The basic principle is that you will lose weight (with diet and exercise) and also if you consume less calories than you burn.
So, keeping that principle in mind, i think what FF is trying to say is that you can overload on calories by not implementing moderation (even with low calorie foods).
A medium sized apple might have about 45 calories. And is a healthier alternative compared to a single 90 calorie chocolate chip cookie. But now if you eat two apples…it’s the equivalent of 1 cookie. See what I mean? Quantity plays a role as well.