I don't think you're in a position to do that unless you can actually solve the problem of grounding scientific objectivity without incurring Munchausen's trilemma. That is essentially an unsolved problem. Analytical philosophy, LW, and various other groups sidestep it by getting together with people who share the same intuitions. But that is not exactly the epistemic high ground.
I'm content to ground behaving as though objective, scientific facts exist in the observation that such behavior reliably correlates with (and predicts) my experience of the world improving. I haven't observed anything analogous about behaving as though objective moral facts exist.
This, too, is not the epistemic high ground. I'm OK with that.
But, sure, if you insist on pulling yourself out of the Munchausen's swamp before you can make any further progress, then you're quite correct that progress is equally impossible on both scientific and ethical fronts.
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: