This is likely misleading for two reasons: about half of Americans say they are trying to lose weight[1], and roughly 90% of people who lose a lot of weight eventually regain just about all of it[2].

So even if there is a mechanism that makes diet sodas counterproductive for weight loss, it's clearly part of a much broader problem. There are also possibly confounding factors, e.g. people most serious about their health and weight loss may have already determined that diet sodas are not good for them.

[1] https://time.com/5334532/weight-loss-americans/ [2] https://healthblog.uofmhealth.org/health-management/weighing...