Bear in mind that they're talking about a specific type of junk food, though, not a healthy diet which is relatively low in fat. The average modern diet has a lot more fat (along with sugar and salt) than is necessarily needed. Also, these are ultraprocessed foods, which as a whole category tend to promote weight gain and undermine health. Such foods typically add lots of fat, sugar, and salt to make them addictive. If the low-fat (or low-sugar, low-salt) foods you choose to eat are fruits, vegetables, and lean meats then you are probably much better off. If you gather or garden at least some of your own food, so much the better, as that outdoor activity promotes health.
If you want to eat commercial foods designed for a certain diet, then read the labels very carefully. A few of them actually are good. I've seen some high-protein, high-fiber, low-carb, low-sugar, etc. products that are tasty and nutritious without a lot of crud in them. But those are hard to find.
For most people, a balanced and varied diet with lots of fresh whole foods is the healthiest option. For those with certain health complaints, cutting out foods that aggravate it (such as reducing sugar for diabetes) or increasing foods that relieve it (such as raising fiber for digestive issues) may help. A problem is that few people have the time, money, energy, or access to such a diet. We'd get better results by fixing the food supply than by nagging people, but nobody wants to do that. Big Food is powerful; individual health clients are vulnerable, especially if caregivers withhold other necessary goods or services in order to coerce them to meet demands.