One's agreement with the claim is irrelevant. Whether one owns a slave or is a slave or lived in the antebellum south is irrelevant. The truth value is the same for everyone. The nature of this claim is exactly like the one above. Only the noun is changed.
People disagree about the truth of the claim, but the claim is objective.
That's why people argue about it. Both of these examples have to do with human beings who are the same everywhere, always and forever. This demonstrates that most moral claims about humans are objective. Subjective claims are claims where the truth of the claim can reasonably differ for you, for me, and for others. The important thing to keep in mind is that we're talking about the actual truth, not just one's belief.