This made more sense after reading the oneshot - titled "Li Mu" - Hara wrote about Ri Boku and Kaine. When I first read Kingdom and got to this chapter, I was at a loss. After reading that, though, things became clearer.
Interesting that the Xiongnu are brought up. They were one of the great steppe peoples, similar to the Scythians and the predecessors to the Mongols. If I remember the Dan Carlin series on the Mongols, in our world the Xiongnu were one of the first steppe peoples to mount a serious invasion of China. Interesting that this Riboku character has apparently wiped them out already! The blonde bastard must be a serious contender.