It does choose a horn, but it's the other one, "things are moral because G-d commands them". It just denies the connotation that there exists a possible Counterfactual!G-d which could decide that Real!evil things are Counterfactual!good; in all possible worlds, G-d either wants the same thing or is something different mistakenly called "G-d". (Yeah, there's a possible world where we're ruled by an entity who pretends to be G-d and so we believe that we should kill babies. And there's a possible world where you're hallucinating this conversation.)

Or you could say it claims equivalence. Is this road sign a triangle because it has three sides, or does it have three sides because it is a triangle? If you pick the latter, does that mean that if triangles had four sides, the sign would change shape to have four sides? If you pick the former, does that mean that I can have three sides without being a triangle? (I don't think this one is quite fair, because we can imagine a powerful creator who wants immoral things.)

Three possible responses to the atheist response:

• Sure. Not believing has bad consequences - you're wrong as a matter of fact, you don't get special believer rewards, you make G-d sad - but being immoral isn't necessarily one.

• Well, you can be moral about most things, but worshiping my deity of choice is part of morality, so you can't be completely moral.

• You could in theory, but how would you discover morality? Humans know what is moral because G-d told us (mostly in so many words, but also by hardwiring some intuitions). You can base your morality on philosophical reasoning, but your philosophy comes from social attitudes, which come from religious morality. Deviations introduced in the process are errors. All you're doing is scratching off the "made in Heaven" label from your ethics.