Nikki Haley isn’t stupid. She knows as well as you and I do that the Civil War was about slavery. So why did she stumble on this issue?
While the mainstream GOP position on racial issues is that the sordid events of the past were just an unfortunate blip with no relevance to today, she knows that a significant part of the base believes that the Civil War was an imperialist conflict forced on the Confederacy by abolitionists with no regard for the rights of states and property owners. She was afraid to offend this group. Unfortunately for her, by refusing to do so, she offended a much larger group and made herself look ridiculous.
If someone put Trump or DeSantis on the spot and asked the same question, you would probably get the same kind of equivocal response, and for the same reason. Fortunately for them, it was Haley who got the question, not them.