True, Plato argued for the primacy of philosopher/kings.
In terms of the argument in Book Nine, however, the passage quoted has more to do with the argument against Thrasymachus who claimed in Book One that justice was the province of the powerful over the weak.

   Then is it not in order that such a one may have a like government with the best man that we say he ought to be a slave of that best man who has within himself the divine governing principle, not because we suppose, as Thrasymachus did in the case of subjects, that the slave should be governed for his own harm, but on the ground that it is better for everyone to be governed by the divine and intelligent, preferably indwelling and his own, but in default of that imposed from without, in order that we all so far as possible may be akin and friendly because our governance and guidance are the same?
   Yes, and rightly so, he said.
   And it is plain, I said, that this is the purpose of the law, which is the ally of all classes in the state, and this is the aim of our control of children, our not leaving them free before we have established, so to speak, a constitutional government within them and, by fostering the best element with the aid of the like in ourselves, have set up in its place a similar guardian and ruler in the child, and then, and then only, leave it free.

Republic Book 9, 590