It's saying that all fat people have tried dieting at some point. Because of this, there aren't any fat people who have never dieted to study. This is presented as proof that diets always fail when really there is no link between the two statements. What purpose is there in studying fat people who haven't dieted? How would a diet succeed if someone didn't first try to diet? The bigger question is why we are asking fat people about whether diets work instead of formerly fat people.