One thing that I think gets looked past in studies like this is the “noob gains” effect. These participants are healthy adults but not highly trained. It’s pretty well known, in cycling at least, that hard interval training is super effective for untrained people or people coming back from a break, but the gains plateau relatively quickly, and the stress of doing this kind of work is hard to sustain for long periods. Another notable thing is that they are doing 45 minute sessions regardless of intensity, in the real world it’s common for lower intensity sessions to be longer, and for those sessions to be a foundation on which higher intensity sessions are carefully added.