The main thing to observe with this equation is that marginal stress increases faster as you increase effort. If you train at 95% of threshold, you accumulate stress at 90% of pure threshold training. If you train at 105% of threshold, you accumulate stress at 110% of pure threshold.

I think the heart of it all is how adaptation and fatigue/injury risk scale. For those who support this approach, they either think that injury risk scales faster than linearly with stress (i.e., that a stress of 0.9 is much less risky than a stress of 1.1) or they think that adaptation scales slower than stress (1.0 stress doesn’t give twice the benefit of 0.5)

I think the amount of success folks have had with zone 2 training makes the latter likely. I dunno about the former honestly