I was recently asking a shop about how EANx21 is done (if someone wanted to put straight air in a tank and then later go back to nitrox), and they said that getting EANx21 from just the air side of a blending system should be simple just by asking for "oxygen compatible air,"but they also said that a problem with asking for EANx21 is that "At altitude you are less than 21% oxygen and to get EAN21 you would have to mix the gas." Is that right? I thought oxygen makes up 21% of air everywhere, pressures just change. Maybe I misunderstood the answer.
You understood their answer. And it seems they understood your question. But their answer was wrong.
You are correct. Air is approximately 21% oxygen regardless of how far up you go. No doubt the shop people were confused by having heard about some of the same issues that have been presented in this thread... but they apparently didn't bother to take the time to truly understand the situation.
If you go to a scuba shop at 5000' and they fill your tank with "oxygen compatible air" you will have 21% oxygen in your tank. If your analyzer does not read 21%, it needs to be recalibrated.