You seem to be missing the practical upshot: If you are middle aged, not exercising, and moderately overweight, HIT twice a week is an effective way to "significantly" improve your aerobic capacity, physical function, and reduce your blood glucose levels. Whether HIT is the optimal way to accomplish that is not the question they were investigating. It's a small sample size but was effective for all eight participants. It works in practice, whether or not you understand why it works theory. There are lots more interesting questions around the edges that can be explored now.