DG’s thesis rests on two key points: (1) the momentary pause at FL230 (about 9 min into the flight) shows that the pilot was alive and well at that point with a properly functioning pressurization system, and (2) the passengers in the back were oblivious to the pilot status for the entire duration of the longer-than-planned flight.
Unfortunately (but typically) he combines a plausible analysis with clumsy drama at 4:08, unnecessarily emphasizing with great certainty that the passengers were conscious and died a horrible death. There was a statement attributed to Churchill where he described John Foster Dulles when U.S. Secretary of State as a bull that brings along his own china shop. An apt description for DG.
Regarding the first point it also seems possible that, given the rapid rate of climb along with the TUC at that altitude, that the pilot would still able to operate aircraft controls and respond to ATC while experiencing the onset of hypoxia. Curious to know if the pause at FL230 was ATC directed.
As to the second point, I find it unlikely that both mother and nanny would sleep, leaving a 2-year-old unsupervised. As to the 2-year-old sleeping, my experience is that, when cooped up in an airplane cabin, sleeping is not among the top 3 potential activities for a 2-year-old. I also find it unlikely that, if awake, the passengers were so engrossed in some unspecified activity, that a slumped-over pilot or extended flight time would go unnoticed until the final spiral to the ground.