Yeah, what Sage is saying is on point. This is a threshold session in disguise because lactate is rarely high and it's cleared quickly. The genius of this session is that you get a bonus running economy/lactate clearance boost. I personally prefer 20-25x400 @10k down to 5k pace with 30-60 second rest or a 200m jog. This is a staple Norwegian workout, and we all know how well that is working at the moment.
Of course, you still need to mix in your standard tempo runs and cruise intervals. A REALLY experienced high mileage guy might do 12-15k of threshold intervals in one session, and you couldn't do that much 5k-10k pace work in one shot. Threshold also works endurance a bit better than 400m repeats simply due to the duration of the reps or continuity of the long tempo.
As to Shorter, he raced a lot (tempos in disguise) and he also probably cruised a lot of his easy runs/long runs. That may not be in the zone we currently define as threshold, but I'd bet he was getting a decent stimulus most days out.