Earlier quoted context omitted.
It's just true enough to not be an outright lie, while being enough of a lie of omission to serve its purpose as propaganda. Yes, the Civil War was about states' rights. Yes, the South was fighting for independence from what it saw as Northern political bias and Federal overreach. But all of those ancillary issues were about defending slavery as an economic and cultural institution.
Saying it was about slavery is also misleading (although less so than just saying states rights). At the time the civil war started, the North was willing to compromise on the issue of slavery [0]. The North did not fight to abolish slavery. Had the South surrendered earlier (or never seceded at all), they would have been able to keep their slaves [1]. The reason the North fought was to preserve the union (apparently…
More accurately, “states rights” was, at the time of the Civil War, a code word for protecting slavery that was revived during the Jim Crow era as a broader code for protecting segregation and systematic discrimination, and the faction which uses it that way has always been strong enough to play a major role in shaping the national mythology.