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.
Everything works. HIT works. Riding your bike once a week works. Walking every day works. Taking up tennis, or soccer, or frisbee works. The point is, short-duration studies on the untrained provide little to no information. It would have been much more interesting to test and compare 3-5 different exercise modalities.
bcbrown covered this pretty well in their response to your comment, but I just wanted to reinforce: it's well established that doing almost anything will show improvements for untrained people. So this study, like most studies, doesn't really tell us anything. On the other hand, many people start exercising, feel great for a few weeks, and then start to not feel so great, and stop seeing improvements. This is the point where we need to start focusing research. There is good anecdotal evidence for what to do next, but it needs to be backed up by studies.
Also, you can hardly claim to know anything quantitatively when your sample size is 14.
