I did read that on the link you had posted. As a former PADI scuba instructor, I do have a few thoughts.
First, I think the need for such a thing would be EXTREMELY limited. Like primarily for Navy SEAL teams and such, and I imagine they already have it covered. I just cannot see a need for underwater combatives. Outside of a limited number of professional/industrial diving occupations, scuba is largely a form of recreation. The vast number of people doing it are there for fun and entertainment, and the very nature of the sport ensures that only those with proper training in scuba are taking part in it. So what I'm saying is: the chance of getting "attacked" by another scuba enthusiast while out on a dive is virtually nil. I just don't believe it will happen. And those wristlocks and leg scissor holds aren't gonna work on the average tiger shark who mistakes a diver for a sea turtle in murkey water.
Second, I believe there was a photo attached to the article, showing a guy applying a wrist/elbow lock simultaneous with a leg scissors across another guy's neck, both wearing full scuba gear, including fins. In applying such a hold, particularly the leg scissors, I can't imagine how the poor victim would manage to keep his regulator in his mouth and his mask on his face. Most likely in a real situation, if someone tried to apply such techniques, either one or both combatants would lose their regulators and drown, if they didn't break apart and recover them. Kicking around someones head with scuba fins would easily dislodge the mask and regulator. That kind of thing can happen if you are simply following too closely behind your dive partner. Seems like that would really be the ultimate fighting strategy for scuba: dislodge his regulator and don't let him recover it or, better yet, use that little utility knife and cut his hoses. Or, dump his weights and grab his low pressure inflator and inflate his BCD and send him rocketing to the surface with a good case of the bends or a lung injury from the expanding air in his lungs, if it happens too quickly for him to exhale. Just hope he doesn't grab ahold of you and take you with him.
My point being that getting into a combat situation in full scuba gear would be extremely awkward and dangerous for both parties. Just a whole lot to go wrong with the equipment and the dangers of being at depth that could cause the fight to end indecisively, or with the demise of both parties. And as I mentioned above, I just don't believe that ANYBODY (Navy SEALS and such excepted) would ever need the skills.
So maybe they use scuba gear simply to have more time underwater to train, with the real idea being that you would probably not actually be using scuba if you needed to use these skills. I dunno what kind of situation that would be. I suppose if I come to Hawaii for a vacation and I'm swimming in the water, some guy might swim out and drown me, but I find the notion to be extremely unlikely. And as I mentioned about the issues with the gear, it wouldn't make sense to train that way if the intended use is actually without the gear. It's just a completely different situation to be in the water with scuba gear, vs. swimming without gear.
When I first saw the link I thought it might be a neat idea. But the more I thought about it, I realized it just didn't make much sense. I think that even training in these techniques could be extremely dangerous.