I don't get why folks often need to reduce such a complex and unique system as our body and its metabolism into some few word description. You are correct, and

I don't get why folks often need to reduce such a complex and unique system as our body and its metabolism into some few word description. You are correct, and there is more, much more: psychology, how our body adapts to training, and so does metabolism. One random example of this vast topic - its well known in weightlifting community if you do hard compound exercises near your limits (squats, deadlifts etc) your metabolism switches to 'higher' burn rate (basal metabolic rate IIRC) and you can basically consume more. One sure way to lose weight if thats the goal, but one should never start weightlifting with those exercises, at that intensity. Also, again generally, its pretty non-smart to take sedentary individuals and expose them to HIIT as the study did, even if its just spinning. Body should gradually adapt to workload (=stress) and HIIT should be the final step in training, since its so taxing. Otherwise injuries and lose of motivation will happen more often than not. The systems that need adaptation are numerous - muscles, tendons, ligaments, cardiovascular system, nervous system etc. People are just looking for quick solutions to problems that don't have ones, not good long term at least. Same as diet fads. The cold hard truth that permanent change of fitness/BMI/overall health for the better requires permanent change in nutrition, training and TBH at least a bit overall life approach. Why not start with that?