No one has denied any of that.
Try to understand this point: the Union ended slavery. That's what they get credit for and that the South seceded from the union to preserve their slavery system redounds to their shame.
And those that still defend the institution over 100 years later are especially pitiful.
“And those that still defend the institution over 100 years later are especially pitiful”
There isn’t anyone defending slavery that I’m aware of.