Why is this surprising? You give someone a major context switch, put them in a structured environment where experts are telling them what to do and doing the hard parts for them (calculating caloric needs, setting up diet and exercise plans), they lose weight. You send them back to their normal lives and they regain the weight. These claims are always based upon acute weight loss programs. Actual habit changes are rare and harder to study. I would expect CBT to be an actually effective acute intervention rather than acute diet and exercise.