As a master Aang can teach other airbenders to improve but can not teach anyone who is not naturally an airbender, even if from the Air Nomads. However, the confusion comes when it is said that it was learned form the Sky-bisons/flying bisons; which implies that anyone can. We can't have it both ways. Otherwise the firebenders, earthbenders and waterbenders could also learn it. Conclusion; You have to be born with the natural ability, and improve to the highest level you can.