I think this is a misinterpretation of what has been debunked. Low fat diets are definitely healthier for more people because they almost always correlate with lower calorie intake. It’s not the quality of it being a fat that makes it bad (this is what recent studies revisiting fats are saying). It’s other qualities of fats that are problematic. For example, consider equal calorie intake between a protein source (like a steak) and a fat. Which do you think is going to fill you up more? Which do you think is healthier?
Fat is calorically dense. If you're trying to lose weight, you need to be careful about eating calorically dense foods. And if you want to eat more protein to maintain muscle while losing weight, there is a zero-sum trade-off between fat and protein.
Interesting archaic theory, as me and my wife just lost 40lbs each by eating a ton of fat and protein, haha. If you want to lose weight, stay away from sugar, for example carbs
High fat high protein (with low carbs) can definitely be successful, as long as you’re mindful of your approach and manage net calorie intake. But for lots of people it is easier to just eat what they do, but make minor tweaks to reduce the amount of fat as a way of reducing calories while still being satisfied by what they’re eating. Different ways for different people.
Weight gain/loss depends on calories, so you can eat anything to either gain or lose weight as long as it's the proper quantity (although if you want to gain muscle then you also need enough proteins).