The only thing about the story that does NOT surprise me is that they were instructors. I am not an instructor or even a divemaster, but I spent some time on one of those islands known as instructor factories, where I heard tales of newly minted instructors "secretly" doing dives that you and I would think unsafe, like doing bounce dives well past recreational limits on an Al 80 "just to test themselves."
This, to me, is the most likely explanation. They did something completely planned, but not what is described in the story. What is described in the story is a fabrication designed to hide the fact that what they planned was so very unacceptable. To illustrate why this sounds plausible to me, I will relate a true story as it played out on ScubaBoard.
Three divers in Cozumel (a shop owner, a DM, and the boyfriend of the owner) had an incident in which they went very deep, had to share air on ascent, and 2 of them were badly bent. The initial story was that they were doing a routine NDL dive and got caught in a horrible down current that took them beyond 300 feet. As the story unfolded on SB, people were swearing they would never go anywhere near Cozumel again if highly experienced divers could get caught in a situation so far beyond their control. People started to call BS on the details (as is happening in this thread), and eventually the truth came out.
The 3 of them had planned to do a bounce dive to 300 feet, with the owner and DM using AL 80s and the boyfriend using an AL 100. At 300 feet, though, the owner kept descending. Two different explanations have emerged--either she was thoroughly narced or she actually passed out, which has been known to happen on deep air dives. Whatever the reason, the DM took off after her, caught her at 400 feet, and got her back to 300 feet, where the boyfriend was waiting. On their ascent, both the owner and the DM ran out of air, and the 3 of them buddy breathed off the boy friend's tank. They of course did not have sufficient air to do the kind of deco required for a dive to 400 feet. As a result, the owner died of DCS, and the DM will never walk again.
Summary: a group of experienced divers planned and executed a dive that was extraordinarily risky. If they had told the truth about the dive, they would have been labeled as truly stupid for doing what they did, so they made up a story that would not make them look so very stupid. People saw through it, though, and eventually the truth came out. Something similar could be happening here.
