Some of the sources you are hand waving towards are (quite rightly) pointing out that rational agents need not converge, but they aren't looking at the empirical question of whether humans, specifically, converge. Only a subset of those sources are actually talking about humans specifically.
(^This isn't disagreement. I agree with your main suggestion that humans probably don't converge, although I do think they are at least describable by mono-modal distributions)
I'm not sure it's even appropriate to use philosophy to answer this question. The philosophical problem here is "how do we apply idealized constructs like extrapolated preference and terminal values to flesh-and-blood animals?" Things like "should values which are not biologically ingrained count as terminal values?" and similar questions.
...and then, once we've developed constructs to the point that we're ready to talk about the extent to which humans specifically converge if at all, it becomes an empirical question..
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: