Does not say that he reported that to ATC, maybe he said that in his statement... Whilst I suppose these things happen and I have seen people severely hypoxic at 8000ft be it with a much higher pressure altitude I find it hard to believe something like this has happend as shown. Even if a heading bug AP has been set the aircraft will climb (if he had his climb power set) and find it's equilibrium somewhere higher then what he originally asked for being 6500ft. As they say the doctors do not know so it is just up to us to speculate but I sense something has been covered up here.