I don't think you're "confused" about what was meant. I think you understood exactly what was meant, and have identified a real (and, I believe, acknowledged?) problem with the moral realist definition of Good.
The assumption is that "if we knew more, thought faster, were more the people we wished we were, had grown up farther together” then a very large number of humans would converge onto moral agreement.
It's not the assumption that is good or bad, but the quality of argument provided for it.
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: