Ordinary moral debates aren't about the meaning of "ought".
I know that, which is why I said "Purported debates about the true meaning of 'ought'" rather than "ordinary debates, which are about the true meaning of 'ought'".
They're about the first-order question of which actions have the property of being what we ought to do. People disagree about which actions have this property.
Please be careful not to beg the question. People agree that there is such a property, but that is something about which they can be wrong.
They aren't stipulatively defining the meaning of 'ought', or else their claim that "You ought to follow the prescriptions of balancing equation Y" would be tautological...
Rather, they aren't trying to stipulatively define the meaning of 'ought', or else their claim that "You ought to follow the prescriptions of balancing equation Y" would be tautological.
In fact, due to people's poor self-insight, time limits, and the sometimes over-coarse granularity of language, they do not stipulate their actual balancing equation. Had they perfect insight and ability to represent their insights, it would be such a tautology. They would cease to speak like that had they the additional insight that for it to do the work it is called upon to do,"ought" is a word that needs grounding in the context of the real reasons for action of beings More generally, they are speaking an idiolect even regarding other definitions.
...rather than a substantive claim as it is obviously meant to be.
It's meant to be such a claim, but it is in error because the speaker is confused about morality, and in a sense is not even wrong. They are claiming some actions have an objective moral valuation binding upon all intelligent beings, but they may as well claim the action has the property of being a square circle - or better yet, a perfect circle for pi is exactly 3, which is something I have witnessed a religious person claim is true.
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I don't understand either why you believe as you do or what good justification you might have for it.
I can see why one might want to make truth claims in which it falls out the folk have the least amount of confusion to be embarrassed about and are least wrong, and if one begins with the assumption that there are "moral facts" in the strongest sense, that's a good start. However, that neither prevents one from having to say they are wrong about an enormous amount nor does it prevent one from having to claim that others, such as me, are wrong about moral facts. I can also see how it might be comforting were moral facts real in that sense and how the world might be better off if people believe in them, but obviously those things would not make them true.
At least you seem to understand the position you are arguing against and I am advocating. You repeatedly have tried to dissuade me from it by pointing out extensions of my position and counting on me to disclaim it from reductio ad absurdum. But it always seems to be by appealing to the same notion, that people simply cannot be wrong about some sorts of things they believe, which is a position I reject.
So, assuming I bite the bullet and don't find it implausible that some people are fundamentally confused when they make moral claims, what other reasons might there be to agree with them? Why should I believe that intensionally defined "objective morality" has any extension at all?
That depends whether you are going to replace objective morality with subective morality or with error theory. It can be argued that subjectivism is an unstable position that amount to error theory.
In You Provably Can't Trust Yourself, Eliezer tried to figured out why his audience didn't understand his meta-ethics sequence even after they had followed him through philosophy of language and quantum physics. Meta-ethics is my specialty, and I can't figure out what Eliezer's meta-ethical position is. And at least at this point, professionals like Robin Hanson and Toby Ord couldn't figure it out, either.
Part of the problem is that because Eliezer has gotten little value from professional philosophy, he writes about morality in a highly idiosyncratic way, using terms that would require reading hundreds of posts to understand. I might understand Eliezer's meta-ethics better if he would just cough up his positions on standard meta-ethical debates like cognitivism, motivation, the sources of normativity, moral epistemology, and so on. Nick Beckstead recently told me he thinks Eliezer's meta-ethical views are similar to those of Michael Smith, but I'm not seeing it.
If you think you can help me (and others) understand Eliezer's meta-ethical theory, please leave a comment!
Update: This comment by Richard Chappell made sense of Eliezer's meta-ethics for me.