I think you are misunderstanding the context of the parent's argument. I'm assuming he's not talking about all states' rights arguments, only those applied to the US Civil War.

If you look at historical documents, protecting the institution of slavery was clearly the main driver for the Southern states to secede. It relatively recent years, Southern apologists have tried to rewrite this as "No, they were just for States' Rights", because this is currently less offensive than saying they just wanted to own slaves.