Maybe you should take a minute and reread the abstract. Using an experimental design, the researchers control for both ROM, salad and what not. Seems you´ve misinterpreted the whole thing. Not only does it show that there are interpersonal differences in the response to different exercise intensities, but even more so it suggests that the are intrapersonal fluctuations in t-level which can be determined by a simple salivary test, and that exercise intensity should be set accordingly so that one would always work at an optimal intensity.
"For as the heavens are higher than the earth,
So are My ways higher than your ways,
And My thoughts than your thoughts."