Interesting point, I've actually thought the same. I think Ultra's is a stretch, I'd say 1500m-Marathon. HM pace is very close to LT, more so for elite men. It might be a larger margin for runners above a 75 min half which suggest as the poster above stated, 10k training (or pace) may be more beneficial.
With the right measures, the phase of specific speed workouts doesn't necessarily have to last that long. I've found emphasis on LT/Tempo work with a short specific phase actually prepares runners best for both tactical and TT races, compared to a lot of specific work leading to poor performance with tactical races.
Emphasizing a lot of LT/Tempo work directly after or toward the latter part of the base phase with the sufficient mileage is basically HM training so it's often an intermediate approach to runners training for XC (5k-10k) in particularly, but even the majority of 5k-10k track runners, and even many strength-based 1500m runners in the winter months.
Non-competitive marathoners with limited capabilities of higher mileage could follow a HM plan and likely be fine as long as they add in a couple 18-22 milers toward the end.