Having read too much history for fun and jobs, the comment sounds wrong. You can go back to the Roman republic to find people concerned about this (Gracchi brothers).
The claim isn't actually that inequality wasn't a topic, the claim is that discussing inequality within the framing that equality might actually be the natural state of thing, or at least just as natural… that was a novel topic at the time of Rousseau.
And sure, it's probably more fair to talk about why even that topic was burgeoning in popularity rather than to imply that it was truly novel.
The claim isn't actually that inequality wasn't a topic, the claim is that discussing inequality within the framing that equality might actually be the natural state of thing, or at least just as natural… that was a novel topic at the time of Rousseau.
And sure, it's probably more fair to talk about why even that topic was burgeoning in popularity rather than to imply that it was truly novel.