Empirical Cycling concludes the same thing on his recent VO2max podcast:
Historically much of the literature suggests that cardiac output , and more specifically stroke volume (SV) is the primary limiting factor
Having a few weeks of 80/20 training under my belt, I can say that it’s not as easy as it might appear on paper, mostly due to having to go HARD! for those hard sessions. It’s not 120%, it’s Hard% FTP. 
The subjects in both groups “ were instructed to perform intervals with their maximal sustainable work intensity, aiming to perform highest possible average power output during each interval session ” (Almquist et al, 2020)…this better reflects real world training, where an interval workout is typically performed as hard as possible, or close to it.
Also wondering if the training/adaptation benefits would be the same if the interval sessions are mixed. Instead of doing 10 weeks of intermittent style, what would result from doing 10 weeks of intermittent and hard-start and long/steady intervals? 