This is contested by a significant number of contemporary running coaches (see, for example "Training for Uphill Athletes" by Steve Johnson or any of the life work of Phil Maffetone.

HIIT is quite effective at something; it's not clear that it is effective at all possible exercise goals (including, but not limited to, endurance running).

Johnson in particular says that unless you've already narrowed the gap between your "first" and "second" (anaerobic) threshold to 5%, HIIT really doesn't make much sense because you're "aerobically challenged" and need to work on that first, which he believes (with significant evidence) is best done almost entirely in zone 2.

He also notes that someone like Eliud Kipchoge can spend 2 hours running 4:30 min/mile pace, so that is clearly within his aerobic range, but that Kipchoge would never and should never spend much time training at that pace because of the load it would put on his system. So the zones that are used for training purposes depend significantly on the current fitness level of the athlete.

I have used HIIT effectively to get myself out of certain fitness/training "ruts", but I think that the zone 2 folks have somevery cogent and coherent observations and advice.