What!? That makes no sense.
They saw an association between sweetener intake and change in fat over 25 years. Not relative to the population, relative to their past selves. How would a weight loss tool increasing your body fat over 25 years be obvious?
They have a pretty detailed discussion section. The main hypothesis they support, based on plenty of other evidence, is that these drugs increase appetite. They motivate you to eat more calories, even though they contain fewer calories themselves.