Craig’s second premise is defended by intuition and his first by the failure of alternatives. Isn’t that an argument from ignorance: “I can’t think how else morality could be grounded, therefore God”?

The argument is an inference to the best explanation, not an argument from ignorance; the difference is whether the alternatives have been examined and found wanting on their merits, which is what the previous three exhibits attempt. The fair criticism is that a fourth alternative is missing here: non-theistic moral realism (Wielenberg, Enoch, Parfit), on which moral facts are basic and need no grounding at all. That is the strongest rival, and it is treated in the Atheism case. The argument holds only to the degree that all four alternatives fall short, and you should weigh that fourth one yourself.