if a God created the universe, then that God's opinions have special moral status

Think about it from a slightly different perspective: the claim is that the universe has morality baked into it -- God created such a universe that moral laws are the same as laws of physics. In other words, the claim is that morality is objective and is embedded in reality. It's not an "opinion" at all.

God does not have special moral status if it had been created by another God, or through evolution

In Christainity (or Judaism, or Islam) God cannot have been created (by somebody else of through evolution). In theology that's one of the biggest differences between God and the world -- one is uncreated and one is created.