You are right that if someone wants to stake the stance that the flag was about state's rights, they need to consider the historical context for what rights they were fighting for.
Context matters. So even if someone takes the flag to personally mean "states rights" to them, okay, that's fine, BUT they also can't deny the context tied to that -- and why that would offend other people. But anyways, I think the war aside, it's still important to note HOW the flag has been used throughout history. It's kind of like how words evolve over time, and stop being used because they become offensive or out of date. The same thing applies to this flag. It's been used as a racist icon/expression for years and year and years. And if someone doesn't want to understand that, I don't know what else you can say to them.