Right, I'm saying moral relativism ends up in inacceptable or at least widely not accepted answers as soon as you apply it to charged topics. It sounded to me like they were making a relativism argument, and I wanted to highlight this - to give them the opportunity to avoid saying that slavery is acceptable (even in the past) - while at the same time warning them against walking into this position unintentionally.
I'm not saying it should be unacceptable, relativism is an interesting position with some upsides, such as that the future, if relativist, will not judge us harshly for our undoubtedly manifold transgressions by their standard, but "slavery was okay actually" is still something that one should, if at all, say with deliberate intent, not as an accidental implication.
edit: To clarify my own view, I think that inasmuch as we now think that slavery was wrong, we have gained understanding - that slavery was just as wrong at the time, at least that it followed from moral precepts that were already believed, but this fact was obscured by the social and economic reality that people lived in. Evidence for this would be that people were already arriving at the view that slavery was wrong based on reasoning that matches, in hindsight, our own.
A good candidate for a similar moral mistake that we'd be making is, of course, the meat industry - meat is tasty and vegetarianism is effort. But I would expect the future to condemn meat-eating for the same reasons that vegetarians today condemn meat-eating, indicating moral progress (or at least technological progress reducing moral effort) rather than value drift.
I'm not sure that morality always works that way, to be clear, but I do think it works that way in these specific cases.