I usually agree with people by saying, "You are correct, it was about States Rights. Individual states wanted to allow slavery. The Federal Government did not."
Now, does the confederate flag mean "I approve of slavery" to everybody? No, it does not mean that, and it probably didn't mean that to everyone back then. To me, as I have said before, the Confederate flag means, "I am an ignorant hillbilly." I might be wrong, but it is my opinion.