This is TR version of progressive overload, you increase intensity or time in zone. TR is focused on time crunched users and goes with intensity. Overall consensus from the coaches about SST is you want to go longer during base, and harder during build (or rather ditch SST and do threshold. FastCat uses 97% of FTP as top of SST that effectively is threshold workout) . There is a years of discussion about TR neglecting time to exhaustion in favour of intensity. But going longer require more time - like doing 2x40 min @90% is 2h workout with warmup and cooldown. This improved a little with recent addition of many longer workouts. But still more intensity to introduce more stimulus is TR way.
Recent discussion about O/U: Overs/Unders - 120/80%? - #60 by spidermonkey
Both styles work, there is some different specificity.