WLC suggests we characterize objective morality in terms of God's nature. In particular, he suggests a form of DCT, that "God's own nature is the standard of goodness, and his commandments to us are expressions of his nature" (On Guard, pp135-6). The obvious reply here is that we can conceive a God whose commandments are morally wrong; so for instance we can envision a God who expresses his nature by commanding, say, a father to sacrifice his son. Clearly this would be an immoral act, and since definitions are true essentially it shows that morality is not defined by God's nature. WLC anticipates this sort of objection, but complains that the envisioned scenario is "logically impossible," on par with suggesting that a square can also be a circle (p136). On his view, of course, that's true enough---but if the contradiction only manifests when we assume DCT in advance, then that just goes to show that DCT is wrongheaded. Since we know human sacrifice is wrong by our moral intuitions, and since we appear to be able to coherently conceive (apart from DCT) a God who expresses his nature by commanding human sacrifice, then clearly morality does not essentially accord with expressions of God's nature.
The solution, then, is not to invoke DCT and thereby beg the question, but rather to show that a God who expresses his nature by commanding human sacrifice is incoherent independent of DCT. Since we cannot define morality in terms of God, we might try instead to define God in moral terms, e.g. to claim that he is moral by definition. However this would prevent us from then defining morality according to God's nature on pain of circularity, hence undercutting the very DCT which WLC wishes to defend. The only remaining alternative in view is to argue without appealing directly to morality itself that God must be the sort of being to behave, ultimately, in accordance with our moral intuitions. But if we permit that sort of move, then we might as well define morality in terms of those moral intuitions directly, without having God play the role of a logical middle man. In other words, by defending against the objection, he concedes a more direct avenue for nontheists to characterize objective morality independent of God.