If it's not a hoax, it's seems like it would be more of a medical miracle than people are realizing? There's way less oxygen at that altitude than at the top of mt everest, plus I've read it got as cold as -65F.
Edit: Tried to explain better. :-(
My extremely uninformed hypothesis is the mammalian diving reflex kicks in since the article mentioned it was cold?????
The wheel well isn't out in the slipstream and probably stays significantly warmer than the ambient temperature, both from having a human in there to heat it and having warm aircraft components nearby. After takeoff, the tires will be hot from hysteresis losses, and the hydraulic fluid will be hot. I'm not sure what other equipment is up in the wheel well for easy maintenance access, but I wouldn't be surprised if the wheel well had several pieces of equipment providing heat throughout the flight. There's also plain heat conduction from the aircraft cabin into the wheel well. The cold also helps survival in hypoxic conditions. Clearly it's not a good idea, but it has been survived many times before.
Edit: http://cdn-www.airliners.net/aviation-photos/photos/1/8/8/17... shows a lot of the equipment in the wheel well, which would have helped keep the well warmer than ambient temperatures.