Second, taking Kirk literally implies that the Yangs now have a moral duty to conquer all Comb territory ...
I was under the impress that what just happened, that was the final battle. The Yangs had won, there wasn't any Comm territory left on the planet.
Any Comms still alive were now in the Yang's single nation. If that is true, then Kirk's words have even more meaning, he is saying that the former enemy is now one of you (the Yangs) and must be treated like you, and have the same rights.
The Comms were now Yangs.