The "partial pressure of O2" has nothing to do with it.

Your premise is wrong. Loss of cabin pressure IS what incapacitates
pilots. The lungs need atmospheric pressure to exchange gasses with
hemoglobin in the alveoli. It doesn't matter what gas it is - without
the pressure, respiration stops happening. As altitude increases and
pressure decreases, respiration becomes less effective. Below a
certain pressure, you can suffocate on 100% oxygen.