Ohh the more I think about it, here's my crackpot theory.
Notice in the quote about king it says "It is believed"
Maybe Rock returns to the peaks, gives the shards to someone to become king, because he doesn't want to.
Instead of that person becoming king and everything is swell, some sneaky person tries to steal the blade/plate by assassinating their kin, this causes conflict obviously.
But then, a greater threat reaches the horneater peaks, like odiums forces.
Rock is forced to protect this assassin/thief person(who is a horneater) from the greater threat and speaks the third ideal (maybe having to do with "even if I have to fight")
Then boom he summons a true shardblade, blows all the horneaters minds, unites them, and becomes king.