I mostly agree with your point, just substitute "genetics" for the actual array of reasons why we have an obesity epidemic. Environment, upbringing, emotional state, level of education, financial resources, access to healthy food, sedentary lifestyle, disordered eating habits, trauma, medications, hormonal imbalances, physical and mental health, etc.

It's common sense that people trying to lose weight are more likely to reach for non-caloric products, and with other studies showing that most people who lose weight will gain it back within 5-10 years, it's makes this study's results obvious and proves nothing new unfortunately. Sweeteners very well could be an independent cause of weight gain, but until they account for all of the confounding factors that influence why people gain and lose weight, they won't be able to determine its true role in the matter.