The fallacy is called voluntarism. Divine command theory (a theory made by humans) commits it just as much as humans do when they make up the laws that violate that every valuer is a valuer who should treat every valuer as a valuer (the golden rule)… on pain of rationality (self-destructs to do otherwise), and the dissonance resulting from failure to align the character of one’s thoughts, values, and behaviors.
An unjust law is no law at all, which is why we don’t need divine command theory. God can create the good no more than they can create their own personhood. They are the good. They always think, value, and behave according to the golden rule that describes them—the cross (and resurrection) being the ultimate demonstration in time.