The American Civil War Was Fought Over Slavery. To claim otherwise is willful ignorance or malice.
Idiots like this man don’t just harm themselves by their beliefs and behavior. They perpetuate one of mankind’s greatest evils, the belief that men are unequal in kind and that a greater man may rightfully hold another lesser man in slavery and use him as he sees fit. Refusing to acknowledge the cause of the Civil War and the evil for which the Confederacy fought is refusing to acknowledge that slavery is a great evil, one that was (mostly) destroyed in the Civil War but continues to live in the minds of men like this.
This refusal to confront and accept the reality of slavery and racism and the century spanning harm it has done and continues to do to human beings the world over makes it possible for men like Hitler and Trump to rise to power. These grotesque caricatures of human beings are acceptable only to those so steeped in hatred and denial that they see the most vile examples of humanity as their saviors, simply because those men promise to grant them the power they feel owed by their race and birthplace.
The state documents of secession are available and specifically call out slavery as the reason for secession. Only South Carolina even tries to be fancy about it, but their document too eventually gets to the core issue – slavery.
If you want to read the documents of secession to see for yourself, or use them in an attempt to enlighten someone, the following is a good source: https://www.battlefields.org/learn/primary-sources/declaration-causes-seceding-states