Now im no chemist, physicist or meteorologist, but i would think air is a homogenous mixture. Meaning oxygen is a fairly consistent 20% (roughly) of the makeup of "air". Im pretty certain oxygen content doesn't change, but the overall air density does.

Even then what he said kinda doesnt make any sense lol. You want to keep the same ratio of a/f no matter your altitude.

The higher you go the leaner you need.

If you had a good tune at sea level and didnt change your settings and say you took a trip (higher in altitude) and ran your engine it will seem rich. But this would also be based on the same ambient temp and relative humidity.

Hope i didnt get too carried away lol.