It seem that Shunsuiju was born,grow up and lived among the Xiongnu for majority of his life(He speak their languages and he is still identify as a one)
SBS doesn't really seem to have much to do with the Xiongnu but him born a Xiongnu seem a plausible headcanon for me,not only his size is very very inhuman(Bananji said that the Xiongnu had inhuman bodies),but his closest friends/vassals were very likely Xiongnu themself.
There should be a reason why apart from Ri Boku, Only Bananji(a man born in the north/fought in the north his entire life) knew about Him.
Batei for example know about both Gyoun/CGR yet doesn't know at all about SBS,and while Kin Mou knew about his name,he knew nothing about his abilities.
So SBS is famous only in the North/Majority of Seika citizens are from the north/Ji Aga and Kansaro are likely frop the Xiongnu....