The study was underweight bmi vs healthy bmi. Nothing about overweight bmi, which I would assume is the trifecta of overeating, little activity, and slower metabolism. At least that is my personal experience in going from overweight to healthy bmi. Really not surprising that people that eat less have lower bmi and higher metabolism than people that eat more with a slower metabolism but who are more active.
Maybe this is not that much of a surprise either though? I wonder how much of this has to do with muscle. BMI infamously ignores muscle mass. An interesting followup would be whether people with low body fat % are sedentary. My guess is that they aren't.
Side note: maybe there is a disparate importance of "Calories In" vs "Calories Out" for people who are under/over weight. Maybe underweight people should focus on exercise more, and overweight people should focus on eating less?