[oarabbus_]

>Participants were randomised to two sessions weekly of high intensity interval training at about 90% of peak heart rate (HIIT, n=400), moderate intensity continuous training at about 70% of peak heart rate (MICT, n=387), or to follow the national guidelines for physical activity (n=780; control group); all for five years.

I can't quite tell by your summary whether you are saying "exercising is good, but the overall effect on mortality is small", or "exercising is good, and the effect of adding HIIT to baseline exercise is small".

You cannot make the former statement from this study as control group were not non-exercisers (and adherence was decent). The latter statement, does seem to be supported.

Regarding exercise in general, the literature shows the mortality gap between exercisers and non-exercisers is absolutely massive.