John Kirksey wrote:

How, then, can someone like Ed Viesturs repeatedly climb the tallest
mountain peaks in the world without supplemental oxygen? Is it because of
the slower ascent and the time spent acclimating to the higher altitudes?

First, acclimatisation.

Second, walking (and climbing) is different from flying. You don't need
much brain to walk. When piloting, however, everything is about
thinking. And it's the brain which needs the most oxygen.

And maybe a third reason: There are exceptional people who are just
better suited for such things than you and me.

Stefan