Even if we make the metaphysical claim that objective morality exists, that doesn't help with the epistemic issue of knowing those goods. Moral realism can be true but that does not necessarily help us behave "good". That is exactly where ethical frameworks seek to provide answers. If moral truth were directly accessible, moral philosophy would not be necessary.

Nothing about objective morality precludes "ethical motivation" or "practical wisdom" - those are epistemic concerns. I could, for example, say that we have epistemic access to objective morality through ethical frameworks grounded in a specific virtue. Or I could deny that!

As an example, I can state that human flourishing is explicitly virtuous. But obviously I need to build a framework that maximizes human flourishing, which means making judgments about how best to achieve that.

Beyond that, I frankly don't see the big deal of "subjective" vs "objective" morality.

Let's say that I think that murder is objectively morally wrong. Let's say someone disagrees with me. I would think they're objectively incorrect. I would then try to motivate them to change their mind. Now imagine that murder is not objectively morally wrong - the situation plays out identically. I have to make the same exact case to ground why it is wrong, whether objectively or subjectively.

What Anthropic is doing in the Claude constitution is explicitly addressing the epistemic and application layer, not making a metaphysical claim about whether objective morality exists. They are not rejecting moral realism anywhere in their post, they are rejecting the idea that moral truths can be encoded as a set of explicit propositions - whether that is because such propositions don't exist, whether we don't have access to them, or whether they are not encodable, is irrelevant.

No human being, even a moral realist, sits down and lists out the potentially infinite set of "good" propositions. Humans typically (at their best!) do exactly what's proposed - they have some specific virtues, hard constraints, and normative anchors, but actual behaviors are underdetermined by them, and so they make judgments based on some sort of framework that is otherwise informed.