Even though it is hard to believe, I actually was a HIT guy for a few years (the way Casey trained was a form of HIT). The original HIT information was good, then it took a wrong turn, with the Heady Duty system for example.
I do believe that the results in the article happened. Dr.Darden is a man worth trusting. But you have to remember that (and Dr.Darden mentions it himself) that most (if not all) of the gains were regained muscle. In fact Casey had been even bigger than he ended up at the end of the study.
I have myself seen such things (not to that extreme) with hockey players I trained. During the season they could lose as much as 15-20lbs. Normally they would regain it in 4-6 weeks of training and eating properly. And the training was very different than the one in the article.
Now, can HIT (or a program like the one in the article) provide results? Absolutely. But you have to understand that it is based on going to absolute contractile muscle failure, not just stopping when it burns and hurts a lot. I'm talking a level of intensity that is hard to do by yourself, you have to be pushed as its that extra effort that make this approach work.
Now how much muscle can it deliver? Impossible to predict, just like with any other system. Everybody responds differently.