The fact that slavery may have been the reason that the southern states seceded still does not make the Civil War about abolishing slavery.
The Civil War still would have occurred no matter what reason the seceding states used for leaving the Union. They could have seceded because they wanted the favorite national color to be green.
Do you think if they seceded because they wanted a different favorite national color from the north that Lincoln would have said "Well, OK then, that's cool. Good luck with your new country, fellas!!!"???