This stood out to me also. I understand why they didn't do so - randomizing exercise for 8,000 people is all but impossible. (Unreliable for "do more", potentia

This stood out to me also. I understand why they didn't do so - randomizing exercise for 8,000 people is all but impossible. (Unreliable for "do more", potentially unethical for "do less"). But it's pretty worrying because there are a stack of possible shared causes between mortality and activity. Everything from obesity (cause of inactivity and shared causes with inactivity) to rarer mortality risks like cancer would simultaneously act to create this result even with no causative linkage. It seems like a new version of that infamous NEJM study showing that low and healthy weights have especially high mortality - because terminally ill patients tend to lose a lot of weight.