Moderation being the keyword, a marathon is definitionally about doing more than moderate exercise

There's a lot of body stress on repairing, maintaining and improving endurance

Maybe that skyrockets cellular division which would maybe would explain some of the impact?

Still.. if you asked me why marathon runners had bowel cancer I would say that they eat more low fiber carbs than is possible for a normal person..

The theory that exercise is harmful seems more like a fanciful wish than a probability given other correlation in environment and behavior where much of our environment is poorly tested and probably harmful.