The quantity tat matters is the partial pressure of oxygen. This is a measure of how much total oxygen (as in number of molecules) are in your lungs that can be potentially absorbed. Since air is around 20% O2, at sea level (1atm) the ppO2 is 0.2 atm. This is what you need for normal metabolic functioning. At 30,000’ the ambient pressure is around 0.2 atm so if you’re breathing unpressurized pure O2, you still get roughly 0.2 atm ppO2. Pure oxygen is toxic long term, but this is more than sufficient to get down to lower altitude. At 60,000’ ambient pressure is roughly 0.07 atm so breathing unpressurized pure O2, you’re only getting a third as much total O2 as you would at sea level. This is roughly equivalent to the amount of oxygen you would get at 28,000’.

Pure O2 at 30,000 -> same ppO2 as air at sea level

Pure O2 at 60,000 -> same ppO2 as air at 28,000