Query re: Partial pressures of O2 in high vs low pressure environments

Hi, I'm currently learning to scuba dive and we use a very simple way to calculate the partial pressure of oxygen at depth to avoid oxygen toxicity, for example at sea level of 1 atm and 21% O2 concentration the partial pressure is calculated to be 0.21, and at 60m (pressure increases by 1 atm for every 10m of depth) that's 7 atm x 21% which is 1.47 which is around the cut off point for O2 toxicity, at which point we start needing to breathe hypoxic gas mixtures to maintain a safe partial pressure of oxygen. For reference the world record dive of 534m was achieved with the diver breathing a mixture of 49% hydrogen, 50.2% helium and 0.8% oxygen to maintain a safe partial pressure of oxygen.

If this is the case for high pressure environments why does the same not seem to hold true for low pressure environments like the top of Everest? If the partial pressure of 100% oxygen at 1 atm is equal to 1.0, why is the partial pressure of 100% oxygen at the 0.35 atm on Everest not equal to 0.35 and is instead hypoxic? I'd appreciate any insight on this.

The partial pressure of 100% oxygen at 0.35 atm is 0.35 atm. Why do you think it is hypoxic?

There seem to be a significant number of reports of hypoxia from people climbing Everest on 100% oxygen, but I suppose this could be because of a limited flow rate I suppose compared to the full flow rate used in diving, or perhaps other physiological issues resulting from the low pressure atmosphere.

Could you link to these reports?
I don't know but suppose they don't use regulators like in SCUBA to equalize pressure of the flowing gas to ambient pressure. Perhaps, a manual flow control instead. (?)

Reading this article seems to explain the hypoxia.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1114067/

"However, it is difficult and expensive to arrange oxygen supplies so flow rates are kept low. The oxygen is used when sleeping, normally at 1-2 l/min via a face mask, and when climbing above 8000 m, normally 2-3 l/min."

Compared to 15-20 l/min when diving a climber would be getting substantially less oxygen into their body even at 100% vs 20%.