Liars can still tell the truth, cheaters can still be honest, and rude people can still be considerate. Just because you can't conceive of a Naturalistic meta-ethic, doesn't mean ethical criticisms from a Naturalist is now invalid.
I don't think DCP can conceive of a plausible metaethical theory he can accept period. He rejects all secular theories and rejects divine command theory. What's left? I don't think he can coherently explain how the proposition "God exists" effectively resolves the problems he believes he is setting up.
That's one of the issues with engaging this kind of argument at the level of providing an explanation for which he claims cannot be explained. I think it does an unnecessary amount of heavy lifting. On top of that, there are as of yet intractable problems in philosophy. The problem of induction remains a problem. I think all metaethical theories have some problems with them, and that's coming from a person who has a definite position on the subject. I think by trying to provide a secular account of moral thought you set yourself up in a position where your interlocutor just provides the secular criticisms of that theory out there in the aether.
Of course, theism doesn't help us develop an account of coherence or moral properties. But that's the heart of the complaint, isn't it? Supposedly, the atheists are in a bad way because they can't explain those issues. This implies that theists aren't similarly disadvantaged which implies that theism helps us explain those matters. I think it is incumbent upon the user of such an argument to go ahead and explain how the theist deals with these matters.
Advocates of the TAG are fond of pointing out that atheists cannot solve the problem of induction and conclude that atheists therefore cannot be rationally consistent and use induction. But when it comes to explaining how theism helps resolve the problem of induction, it ends up in answers like, "God is lawlike, so it follows from God's nature that the world is lawlike," or "God is good, so God would create a world that can be understood by his creations therefore the world is expected to be regular on the hypothesis my God exists." And for a variety of reasons those answers are far more pathetic than what goes on in real philosophy. I think that's where the focus should be.