Wow. According to the article he was unconscious for most of the flight, which is not surprising. What IS surprising is that he was found to be unharmed. My conjecture is that this is something similar to when people "drown" in very cold waters and are able to be revived after relatively long periods underwater. The temperature at that altitude is about 50 degrees below zero.
Also, according to the Google Books scan of "Jar Private Pilot Studies" by Phil Croucher and Staff the partial pressure of oxygen is low enough at 22,000 feet to cause death.
Here is another similar story from 2000:
Survival of this practice, however, is not the norm.