I don’t know exactly what they did in the study, but what I would expect is that you want to be doing the entire range of intensities, but in the off-season not necessarily to exhaustion.
I am currently following a 10hr/week training program from Join and from what I have seen so far, it has me doing 3 workouts per week with blocks of low endurance at 60%-70% and high endurance at 70%-80%, and the other 2 workouts are also endurance with a few 5-10min tempo blocks in the middle on one day and 1 minute all-out or 30-15’s blocks the other day. Not much threshold work so far.