I think this is about metamorals not metaethics - yes, I'm merely defining terms here, but I consider "What is moral?" and "What is morality made of?" to be problems that invoke noticeably different issues. We already know, at this point, what morality is made of; it's a computation. Which computation? That's a different sort of question and I don't see a difficulty in having my answer depend on contingent facts I haven't learned.

In response to your question: yes, if I had given a definition of moral progress where it turned out empirically that there was no coherence in the direction in which I was trying to point and the past had been a random walk, then I should reconsider my attempt to describe those changes as "progress".