Avic, you actually don't want to use nitrox when going deep, it's used to extend dive times but not a special mix for going deep. Oxygen becomes more toxic with increased depth, thus you don't want a mix with higer O2. Tech divers that go way down use trimix (a helium blend), dives down to 150 would more likely be on regular air. 130' is the rec limit, but maybe these guys were trained beyond that. Being relatively young, maybe this wasn't the case, but it's very hard to say. No reason an 18 year old can't be a well trained tech diver.
Any dive site can be dangerous if you go beyond your limits. Whytecliff is a beginner site, that is where all the training is done. The "Cut" at Whytecliff, which is where these divers were, is around the bend and not used for training (at least not the open water classes). It's a more difficult dive as it's a wall that drops down a long way so your buoyancy needs to be better.