The sultan: the proof, and his speaking. It is a metaphor, as you say: his book speaks of such and such, and this is what the Qur'an has spoken of. Its meaning is: the indication and testimony, as if he said: it testifies to their associating partners and to its truth. And what is in بِما كانُوا is a source meaning: by their associating partners with Allah. It is also possible that it is a relative pronoun and the pronoun refers back to it. Its meaning is: he speaks of the matter that causes them to associate partners. It is possible that the meaning is: Or have We sent down to them a sultan, meaning: a king with proof, and that king speaks with the proof by which they associate partners.