There were plenty of Westerners who were concerned with inequality -- there were numerous failed peasant rebellions throughout Europe ever since the Middle Ages.
But what he addresses is radical equality, as a fundamental value, as opposed to "less inequality". Equality under a ruler or rebellion against abuses is not the same as thinking the whole society as composed of equals. Think of no-slavery, equality among genders, sexual freedom, etc. Even thinkers that thought about equality before Iluminism (e.g. Thomas More) didn't go that far.
I strongly recommend you reading, at least, chapter 2 in the book. Is way deeper than what I explain.
Typical Graeber. Total nonsense. Radical equality has multi-origins in religious cults for 1000s of years. It has been part of 'Western' history writing for almost as long as we have historical narratives.
Pretty much as far as we can tell these ideas are resurfacing over and over in every big culture. For most we have no documentation but its clear there and has been documented lots of time.
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.
Also, it is quite ironic to mention the thought of Rousseau as an exemplar of the Enlightenment when he is often put into the camp of Counter-Enlightenment.