I don't think the author grasps it either, especially when he uses morality as an outcome rather than just another cultural value. Assessing the quality of the outcome requires a cultural perspective itself, and even if from that cultures perspective the outcome is poor, the values that the culture would have to change may be worse.

If my culture does not accept that slavery is acceptable, but is aware that it will be wiped out by the cultures that do allow it, and chooses that fate, my culture is not inferior for doing so from my perspective.