@Fangstrike: The way I saw it, the fate of the battle rested on Kouma vs Kiri, since they were essentially each sides best. In canon Kouma won and then tore through the rest of the Nanaya clan. In this fic Kiri won and then the Nanaya washed of the Tohno like a wave.
I was going with the interpretation that the Tohno sometimes produce beasts like Kouma but the Nanaya have a more sustainable level of competence spread over the whole clan.