I dare say that a disembodied, solipsistic mind wouldn't need to think much about morality. But an embodied mind, in a society, competing for resources with other agents, interacting with them in painful and pleasant ways would need something morality-like, some way of regulating interactions and assigning resources. "Social" isn't some tiny speck in mindspace, it's a large chunk.
There seems to be a widespread impression that the metaethics sequence was not very successful as an explanation of Eliezer Yudkowsky's views. It even says so on the wiki. And frankly, I'm puzzled by this... hence the "apparently" in this post's title. When I read the metaethics sequence, it seemed to make perfect sense to me. I can think of a couple things that may have made me different from the average OB/LW reader in this regard: