They also seem not to have controlled for terrain. High altitude locations are, generally, more mountainous. That means, even without any increase in perceived exercise, you are doing more vertical as you go about your day.
I grew up in the mountains. When I went to basic training I was all set for serious hiking. But basic training wasn't in the mountains. I was shocked at house easy long marches were when there were no real hills.
A surprising number of calories are spent repairing skin damage. Increased uv exposure at altitude probably makes a difference, but probably doesn't help skin cancer rates.
I grew up in the mountains. When I went to basic training I was all set for serious hiking. But basic training wasn't in the mountains. I was shocked at house easy long marches were when there were no real hills.