[Ives]

An important part:

> This is an observational study, and as such, can’t establish cause. And the researchers caution that the number of included studies was small and their methods varied considerably, which may have influenced the results.

People who are sick or have other health issues will do very little running, so correlation and causation and all that.

[MaximumYComb]

Similar to how a low BMI has higher death rates than overweight BMI IIRC. People tend to lose a lot of weight when they are really sick.