I have not checked all the media stories but the assumption is that the a/c flew above 30,000 feet - well and truly in the hypoxic zone .
Is this a correct assumption ?
Maybe the a/c flew much lower than that though the flight would have been round 1000 odd miles.
People have survived at high altitudes - for example , people have climbed Mt Everest ( 29, 000 feet ) without additional oxygen -
https://en.wikipedia.org/wiki/Reinhold_Messner
The other issue may be that this man had lived for a long time in an elevated region where oxygen levels were low and he was acclimatised.
Did he smuggle an oxygen cylinder into the wheel chamber with him ?