This study directly tested whether an activity tracker is a beneficial addition to weight loss counseling.
No, that's not what they did. That's what the OP is complaining about, there's no control group.
All participants were placed on low-calorie diets, prescribed increases in physical activity, and received group-counseling sessions on health and nutrition.
...At the first six-month mark, participants were divided into two subgroups: one that continued health-counseling sessions on a monthly basis and another that received a wearable device to monitor diet and physical activity.
...However, those who received health counseling throughout the study lost nearly twice as much weight as those who used wearable devices for three-quarters of it.
The group that got fitness trackers also stopped participating in the health-counseling sessions.
The conclusion that people using fitness trackers lose less weight isn't valid. Maybe the health-counseling sessions caused people to lose more weight? There should have been a group of people that got both fitness trackers and counseling.