For the Songnu and Sirong tribes Aeryon did not enslave them but instead ask them to join his tribe.
Most of them feel closeness to the people of the Great Steppe and so they joined him willingly.
But the more important reason is because they respect the strong.
Strong in the minds of the tribes of the nomadic people inside the Wall of Yan does not mean simply that you know how to fight.
If that is the case most tribe should actually be ruled by people with great strength and those enthusiastic warriors.
Strength of a chief is not measured on how many enemies they could kill.
It is how many people of his tribe that he could raise, he could save and that he could protect.
And in the eyes of the Songnu and Sirong clans that decided to surrender to Aeryon, they could see that Aeryon could raise so many people, cultivate great warriors, possessing order and discipline that is akin the armies and soldiers of Yan.