That's no theory of why endurance exercise increases reserve capacity in most people. It's jut an observation that this frequently happens. It not even good enough to tell you the optimum duration of your exercise.

I also don't happen to be an believer in the idea that VO2max is the best possible value that one could come up with, when it comes to issues like surviving illnesses.

It took it quite a while to figure out that keeping medical ventilators at a static pressure level is a pretty bad idea. A static pressure level is really straightforward. With better research I think it's likely we find a more complex variable that does a much better job than VO2max.