One has to be careful about these sorts of findings.
I do not think they made any attempt to control for the fact that people who are healthy are able to exercise. The habit of fitness may be a result of health and not the cause of health. Really sick people do not exercise because they can't.
I would think that a healthy life style would be a to one's long term health. But you can get a pretty large effect in a statistical study by just inadvertantly including unhealthy people in one group and excluding them from the other group.
That sounds like a good subject for a follow-up study. And you're right, of course.