I WAS THERE! How David Blaine flew helium balloons to the height of jets (and jumped)

1:31 and 1:56 - What you and David said isn't "wrong", but the way you summarized the death zone may not be clear to others.

It's true that humans can't survive above certain elevations/altitudes without supplemental oxygen, but it's not a magic point where you suddenly can't breathe anymore and will die after, say, 1-3 minutes -- which is what I think it kind of sounds like in the video.  The "death zone" is actually a point where you can survive for a time (depending on how well you've acclimated, of course), but your body actually shuts down processes that aren't immediately necessary to survival -- like digestion -- due to that lack of oxygen.  (You don't just suddenly stop breathing.  I know you didn't say that, but I think it may come off that way to some people.)  Climbers making a summit run on Everest typically don't even try to eat in that region, because your body can't process it anyway.

So you don't die instantly (or in a few minutes), but it's true that the clock is ticking, and you only have so much time before you need to be back down far enough that your body can resume normal processes.

For someone in good shape who has trained for it -- i.e., most people attempting to climb Everest -- they typically have several hours they can survive in the death zone, at least as far as oxygen is concerned and as long as other medical problems don't pop up (e.g., a pulmonary edema or something).  And, as you definitely mentioned, cognitive function is greatly decreased, so that can lead to making bad or downright bizarre decisions, which can also obviously lead to disastrous results.

As implied in the video and earlier in this comment, the specifics vary from person to person, and even situation to situation.  Factors include: your personal biology, how well you've acclimated during that trip, hydration, nutrition leading up to the summit run, how hard you push, clothing, weather, anxiety… pretty much everything.  Yes, Sherpas tend to be better prepared to deal with that, partially because they've lived at higher elevations for many generations, and partially because the ones aiding climbing expeditions tend to have a lot of experience actually doing things at more extreme elevations.  But non-Sherpas have made it to the summit of Everest and back without supplemental oxygen, too; it's just less common.