This weakens the author’s argument by bringing in a third factor: diet. Perhaps the people who didn’t die were saved by their diets rather than by exercise.

This doesn’t impact the argument without more information. For example, we don't know if the groups actually had different amounts of time available, and if so, why.

This undermines the author’s argument by indicating that people died because of their preexisting conditions rather than their lack of exercise.

In other words, there was no connection between prior health and exercise. This defends against the possibility that only already-healthy people were exercising, which strengthens.

We don’t care what usually motivates people to exercise. We care about why the study showed a difference between people who exercised and people who didn’t exercise.