As clarified in the other reply by the other commenter here https://news.ycombinator.com/item?id=29195026

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.