The columnist doesn’t fail to consider this possibility. She probably knows that other factors like diet or smoking affect one’s risk of cardio-respiratory illness, but she’s only discussing the effect of strenuous versus nonstrenuous exercise.

The columnist is only arguing that vigorous exercise reduces one’s risk of cardio-respiratory illness. Whether it also increases one’s risk of physical injury is irrelevant.

The columnist’s argument only addresses the effect of vigorous exercise on one’s risk of cardio-respiratory illnesses. Whether vigorous exercise also prevents other diseases is irrelevant.

The columnist is discussing the actual effect of vigorous exercise on one’s risk of cardio-respiratory illness. Whether people who exercise vigorously perceive themselves to be healthy has nothing to do with their actual health.

The columnist assumes that the recent report is correct and the older studies are wrong, but she fails to provide any evidence for this. She doesn’t give any reason to believe that the conclusion of the recent report is better justified.