Craig's Moral Argument Refuted?

Recently, it was brought to my attention that a YouTube video claims to have debunked William Lane Craig’s first premise in his moral argument. For a refresher, that premise claims “If God does not exist, then objective moral values do not exist.” The objector claims that if even one of the Euthyphro dilemma’s horns are even possible, then the first premise is false. This is (likely) due to an understanding of objective moral values (and/or God) as necessary. Since God is posited as the objective grounds for objective morality, if there is a possible world in which objective moral values exist outside of God, then God is not the grounds of that morality. Moreover, if there is a possible world in which God decrees objective morality, then the premise is false.

In evaluating the second part of the dilemma first, I am baffled as to how anyone can affirm this and still deny the first premise. The only way one can deny this conditional is by affirming the antecedent (“God does not exist”) and denying the consequent (“objective moral values exist”). But if God exists, then by definition the antecedent is not affirmed. Instead, what people have typically done with this horn of the dilemma is deny objective moral values exist, not deny the first premise.

Now the first part of the dilemma is possibly true. It could be true that objective moral values exist and God does not exist. The problem? One needs to show why we should think that it is more likely that God does not ground objective moral values than that he does. “Now wait” the objector may protest, “Have you forgotten? If one of the horns is even possible, then the first premise is false, because then God does not ground objective moral values in every possible world, and hence he does not at all.”

Yet one must consider that the reverse is true: If God’s grounding of objective moral values is even possible, then that horn of the dilemma is impossible! This is because of metaphysical possibility (not mere epistemic possibility). Epistemic possibility states “for all we know, such and such is possible,” while metaphysical possibility deals with what is really possible, after all. In this case, we’re at a metaphysical standoff. But this claim was supposed to be an objection to the first premise; it can’t get off the ground without assuming the first premise is false. It must either be withdrawn or supported by other arguments that will, in all probability, be less likely than God’s grounding of morality or rely on God’s non-existence (or both). This utterly fails as a functional objection.