I doubt that metaethics can wholly capture the meaning of ethical terms, but I don't see that as a problem. It can still shed light on issues of epistemics, ontology, semantics, etc. And if you want help from an AI, any reduction that gets the reference correct will do, regardless of whether meaning is captured. A reduction need not be a full-blown normative ethical theory. It just needs to imply one, when combined with other truths.