It defines the oxygen requirements in terms of density altitude.
Not really. The body only works at body temperature, so the air temperature in the lungs is body temperature, so pressure is proportional to density
in the lungs. Not in the atmosphere, standard or otherwise.
It makes much more sense to work in pressures. The Saturated Vapour Pressure of water is a pressure. The partial pressure of CO2 in the blood is a pressure. The "A-a" gradient is a pressure.
Google "Alveolar Gas Equation".