If being human is being moral (correct me if I am misrepresenting the concept)
It isn't that being human is being moral, it is adhering to human telos.
A knife that cannot cut anything is still a knife, it just isn't a good one (in fact, it is objectively bad).
then were people 2000,
Fun fact, the telos model of morality/virtue ethics (which is currently the most accepted model by ethicists) ultimately is derived from Aristotle's morality (it is neo-Aristotelian virtue ethics).
4000...10,000 years ago bad humans because they didn't share all of our moral precepts?
Not at all. People still tried to be virtuous back then, and moral progress is possible. Nothing about moral realism prevents moral progress from also being a thing. The issue is that we have limited knowledge, and as such the further back in time we go the less advanced the knowledge we can expect to see.
We don't look at today's knowledge about gravity and then say Newton was bad at science because he was wrong.
So to with moral advances. Yes, we know that what they conceived of as being moral was inaccurate, but within the framework of what knowledge they had access to there were many moral people.
They weren't bad, they were mistaken.
Just as we advance in other fields with increased epistemic access, we progress morally as well.
It should also be unsurprising if we find out that the people today that we find to be the most moral would be looked back upon in a thousand years of holding immoral beliefs of doing immoral actions, but that isn't a slight against the people today. We must act based on our current knowledge.
EDIT:
I don't understand the 'telos model'.
I do apologize if I am not explaining it well enough. As I said earlier, I am still not too well read in this field. I have dedicated some time to it, but have a lot left to do before I would even be confident enough to do a debate on ethics.