The above poster is - as I understand it - pointing out that this study does not necessarily contradict the hypothesis you mentioned. A small amount of exercise is shown to be beneficial, but it is entirely possible that larger amounts of exercise are detrimental.
Do you know if any research has been done into that hypothesis? Because - failing a confirming study - it does not seem to fit with anything I've heard. I mean, maybe some organs can get overwhelmed if metabolism is pushed too high too quickly, but in general I doubt that a fast metabolism even correlates with lower life expectancy.
