Frankly that reads like a conspiracy theory from someone in the middle of a crisis. I highly doubt that even if real it has anything to do with thin-obsessed female minors.
It is a legitimate growing concern for men's fitness content and the (young) men who watch those. But if their focus is on women's thinspo stuff (given the title concerns both weight and fitness), that could be a "one stone, two birds" situation, assuming the exposure restrictions work.