I haven't measured my blood oxygen, alas.

I've been here 4+ years now, and other than being 4 years old and just coming out of a sedentary 3 year period, I notice zero effects of living (and running, cycling, swimming, hiking) at altitude. Can't say any more than that really.

Hmmm, given that...it does seem to me the initial claim is likely to be true.

I know you said that it has to vary between multiple individuals at altitude, but it's unclear to me why.

Don't we just have to show some individuals don't ever get back up to the same blood oxygen levels?

Well, another comment here from a guy of Sammi ancestry notes potential polymorphism in GHC1 as possible (and likely) partial explanation.

I would imagine that there are a variety of potential genetic differences that could generate a wide range of altitude-adaptation differences.

Isn’t Apple Watch and other blood oxygenation just measuring saturation (a relative measure)?

Among other things, Acclimatization involves producing more blood-oxygen carrying capacity (or less if you go back to sea level), which is an absolute measure, not a relative one that the oximeters would pick up.

Yea, we would only need a small percentage of folks unable to acclimate to create a significant correlation.

I can only speak anecdotally about my experience when I was young and now that I am much older. When I was young I went from about 69' MSL to about 6780' MSL when doing a job at the Air Force Academy and we dared each other to run up some stairs. We made it about half way up and fell over laughing and gasping for air. Our team chief that smoked about a half a carton of cigarettes a day walked right past us shaking his head. For about a week or so I had some pressure in my leg near the hip but it cleared up.

I am much older now and a couple years ago I retired/moved from about 600' MSL to 6300' MSL and it took about a half of a year for my body to adjust including some pain in my legs with some associated popping and other weird pressure like feeling similar to what I had when younger but it took longer to go away. Probably too many decades behind a computer and not enough walking. I've had no feelings of self defenestration.