Saying free-floating moral facts anchored in no mind are hard to account for applies equally to the theist’s view that God’s nature is good without further explanation. Wielenberg makes this parity argument explicitly.

The parity argument is Wielenberg’s central move: if the theist can stop at God is good by nature, the atheist can stop at certain moral facts are basic. The theistic reply is that a mind is the kind of thing that can be a source of obligation, since obligations are owed to someone, whereas abstract facts are not; so the theist’s stopping point explains the bindingness of duty and the atheist’s does not. Wielenberg has a response to that too. This is a genuine debate, not a knockout, and the exchange is worth reading in full.