At one point they admit that using four subs they got significantly less output than two, albeit smoother response (paraphrased). I would argue that the same is true for two vs one in a home theater environment (reasonably sized room). Their tests seem aimed at getting the smoothest response over a atypically large viewing/listening area, and in only a medium sized room at that. I would presume that most people don't use multiple rows of seats in their home theater (6 foot by 6 foot). One row spanning ten feet or so seems more likely. It is too bad that they mention how important sound treatment in a room can be and then just blow it off and move on to adding more subs...too bad!!!
Just don't think people should think throwing a second sub in a room will solve all their bass problems, hardly!