Without commenting on whether this presentation matches the original metaethics sequence (with which I disagree), this summary argument seems both unsupported and unfalsifiable.
Would this be an accurate summary of what you think is the meta-ethics sequence? I feel that you captured the important bits but I also feel that we disagree on some aspects:
V(Elves, ) = Christmas spirity
V(Pebblesorters, ) = primality
V(Humans, _ ) = morality
If V(Humans, Alice) =/= V(Humans, ) that doesn't make morality subjective, it is rather i...
Unpacking "should" as " morally obligated to" is potentially helpful, so inasmuch as you can give separate accounts of "moral" and "obligatory".
The elves are not moral. Not just because I, and humans like me happen to disagree with them, no, certainly not. The elves aren’t even trying to be moral. They don’t even claim to be moral. They don’t care about morality. They care about “The Christmas Spirit,” which is about eggnog and stuff
That doesn't generalise to the point that non humans have no morality. You have m...
Morality binds and blinds. People derive moral claims from emotional and intuitive notions. It can feel good and moral to do amoral things. Objective morality has to be tied to evidence what really is human wellbeing; not to moral intuitions that are adaptions to the benefit of ones ingroup; or post hoc thought experiments about knowledge.
In a sense, the ancient Romans did believe this. Anyone who ended up in the same situation - either taken as a war captive or unable to pay their debts - was liable to be sold as a slave. So what makes you think your position is objectively better than theirs?
This assumes without argument that "value" is something people intrinsically have or can have. If instead you view value as value-to-someone, i.e. I value my loved ones, but someone else might not value them, then there is no problem.
And it turns out that yes, most people did not have an intuition that anyone has intrinsic value just by virtue of being human. Most people throughout history assigned value only to ingroup members, to the rich and powerful, and to personally valued individuals. The idea that people are intrinsically valuable is historically very new, still in the minority today globally, and for both these reasons doesn't seem like an idea everyone should naturally arrive at if they only try to universalize their intuitions a bit.