Does work in terms of boosting stamina and T-levels. Many different ways to arrive at the end goal. Sprints, Running up stairs, Walking Lunge-Lunge-Squat with heavy dumb bell or large plate, Burpees to failure, etc.etc. Pick your poison.
He is referring to High Intensity Training, not High Intensity Interval Training.
As my name implies, I am a fan of HIT training. I believe that Mentzer was way ahead of his time in particular. I'm not saying he was right about everything, but he developed theories that you could apply and measure, which is a big part of this entire process that most programs lack. i.e. - this is what works, if you try it and it isn't working than you have a dosage issue (too much or too little) and the theory allows you to adjust to your needs.
Having said that, other forms of training seem to work too, and I don't follow Mentzer's, Darden's nor Jones' routines exactly as they prescribe. When I have followed their routines and when I have applied their principles to my own routines, I have seen consistent progress at a much faster rate than most people experience. Having said that, the same can probably be said by certain people who use German Volume Training.
I think that some people are mentally geared toward working out at lower intensities for longer durations and others are geared toward high intensity for brief durations. I happen to like the all-out effort and rate my workouts on how far I "pushed". i.e. - if I got to a point that I thought I was done and got an extra 1-2 reps after I had "given up" - then I know I went hard. If I then do a drop set and repeat that 2-3 times without a rest, I will be fried and know I had a good workout that will produce results next week.