The trope that the Civil War (War Between the States) was only about slavery is certainly incorrect. The issues were complex. You are correct that the issue is not whether Haley's gaffe alone makes her unfit to be POTUS. What is the issue is that her comments reveal that she is catering to a certain demographic on the far-right that believes in a revisionist history not simply about the causes of a war but about the meaning of the nation itself and its history.