Marcus_Adderley
I guess his origin must be that his airbending was a recessive gene. If you think about it, in order for no more airbenders to be born outside of being the descendents of a single man, it would imply that no airbender or child of an airbender throughout history has ever left the air temples and settled in a differed land. But that is almost completely unreasonable to expect. While you could say that the technology of the past did not allow most people to go to far away lands and settle there, that was never the case for airbenders who had flying bisons to travel to any location in the world. No other nation could so readily transport people across the map. As for why the hidden airbenders did not reveal themselves, that would be because they did not know that they were descended from air nomads. If any air nomad survived the genocide and had children, they would have done everything in their power to save their children, including not revealing their air nomad origins to them. That would be even easier if the children were not airbenders in the first place. In the avatar universe, it seems like bending is a recessive gene which means that benders are more likely to give birth to more benders if they have a child with another bender. At the same time, nonbenders are more likely to give birth to children that are nonbenders. But there are always exceptions and that is what happens when benders have nonbender children or vice versa.