Did this a/c have a built-on oxygen system, or was it a portable kit, perhaps with demand regulators?
I have flown up to 20000ft with my portable system, with cannulas, and above about 16k one needs to breathe fairly decisively. Above about 18k, more so. At 20k, very much so. Cannulas are not recommended above 18k anyway.
I haven't seen a report on this incident where it says how high he actually went but it is quite possible that he was not breathing properly e.g. using his mouth rather than his nose (if using cannulas). Also some people are affected much more than others; I once flew with one chap who was affected badly enough to not be able to read the altimeter after about 30 mins at FL120. Within seconds on o2 (he had earlier refused o2) he was fine. I use o2 anywhere above about 9k; at those low altitudes the flow rate is very low and it keeps one wide awake.
During the ground checks,I managed to find one crumpled,and brittle plastic mask,and a non-working oxygen system.Yet the aircraft was regularly used,and with passengers in areas that had safety altitudes above FL150...!
Why am I not suprised?