Plenty of people lost weight on low fat diets too when those were the hot shit a decade or two ago.

The problem with fad diets is that most of those people gain their weight back after the initial enthusiasm fades.

Without long-term (decades) follow up, it's impossible to distinguish the effect of a particular diet from the impact of simply being on a diet of any kind--which typically induces much closer attentiveness to calories and nutrition, at least in the beginning.

Also, Internet forums are hardly a random sampling.