Harris' point is that there is often more than one good answer to ethical questions - peaks of equivalent or similar altitude in the Moral Landscape.
Keep off the rock and respect the wishes of the owners, is one answer that really does little harm to anybody and a lot of good for the dignity of native Australians. It might be as good an answer as any.
Leaving fallen climbers to die on the rock in order to respect the supposed sacredness of the rock would probably not be a equivalent peak.
If you didn't get that much then you didn't actually read the book.