Well, Novak may tell it like it is, but whoever wrote those articles seems a bit confused. First of all, it is NOT a hyperbaric chamber--it is a hypobaric chamber. It does not increase atmospheric pressure--it decreases it, simulating the effects of high altitude. It would seem to me that any benefits from it would only occur after fairly long-term use, the same as altitude training, as the body undergoes physiological changes to deal with reduced oxygen and air pressure. "Altitude tents" (which this seems to be a fancy version of) allow athletes to reap the benefits of sleeping at high altitude and then training at lower elevations (wherever they are actually training). As I understand it, "sleep high, train low" is the ideal way to benefit from high altitude training. I don't know how much time Djokovic spends in it, but I wouldn't think that spending just an hour or two here and there would do much of anything (unless he believes that it does!).
Looks cool, though!