guided cavern tours in the Akumal area are highly organised and even federally regulated (SECTUR NOM 9). Many many thousands of guided cavern dives are conducted without incident every year. In fact, I would go as far to say, that the cavern/cave dive industry in this particular region is one of the most professional and safety conciencious dive industries in the world (and I have seen many of them)
THe Dos Ojos system, while it is one of the largest cave systems in the world has so much cavern zone that it can seem that you go for miles, when in fact you are never more than a few meters from breathable air.
Cavern divers often think they are in the dark when, if they covered or turned off their lights and let their vision adjust they would clearly be able to see daylight from the entrance.
Angelita is basically a vertical sink hole until the rubble pile below the layer at 40 m/130 ft, however there is a large tunnel that goes from there to slightly less than 200 ft. As long as you are comfortable in the 100-130 ft range, going below the layer is no problem.
I dont think this guy was advocating anything dangerous. He was on a guided dive in one of the most stunning dive sites in the world and was raving about it.
For anyone who thinks you can just rent tanks and jump in these holes in the ground, I think you will find it difficult to impossible to do so without some kind of overhead environment cert card or a guide..