DG, interval training and HIIT are not exactly the same, but they can cross over in a lot of areas. Typically, in interval training, you have varieties of sprints/hills, and they may or may not bring you to 95% of max HR. The rest intervals may vary. The sessions may last up to an hour.
Occasionally, there is some crossover, though, that makes the line very blurry. For example, I do one interval session that in the beginning involves 10 30 second sprints with a one minute rest in between. That part is straight HIIT, except perhaps shorter. But then the session goes on for another 40 minutes with longer intervals at a lower intensity.
I hate to even comment on HIIT since I caused a stink about it awhile back, but, just as an observation... I've seen this explanation alot, actually multiple times from you, and it seems awkward every time. At least from my experience, when you get into athletics, it actually pretty much is the same thing.
I ran track for 10 years or so, including a state record, and including college. I played football for 9 years. I have been racing mountain bikes for 8 years now, and have also dabbled in the road biking scene. My wife is also a very avid runner, she just got something like 121st out of about 14,000 women in the Nashville half marathon, so I hear all kinds of stuff from her as well, and her experiences in track. Between the two of us we have read all kinds of books and magazines about running, racing, biking, or whatever.
Even after all of that, except for seeing it mentioned online, actually mostly just here... I cannot remember ever hearing the term HIIT at any time in my life. Even during the present time, we talk about intervals on the bike ALL the time, yet HIIT has not come up one single time. It's simply "intervals". At least from the background that I'm from, if you're doing intervals... you can bet your ass that they're high intensity. It's just both interesting and strange to see people trying to distinguish between normal intervals and high intensity intervals. Based on my experiences it just seems like it should be the opposite if anything else, and call the slower set "low/medium intensity interval training", because that kind of efforts and intervals just don't go together. If you don't believe me, go to forums.roadbikereview.com where alot of serious road bikers go, and most of them do intervals, and do a search on HIIT. Only 3 threads come up, and even then it's just mentioned in passing like once in the thread. Compare that to this forum, 260 threads, and it's just non-stop chatter about it. It's just kind of strange.