"he was really angry that he saw what was happening with regard to the Civil War."

So that's the part that is important, not "been a little bit later, you wouldn't have had the Civil War."or your quote?  Trump is guilty of speaking incomplete sentences and letting his thoughts run together, nothing more.
BTW - Abraham Lincoln was not a proponent of slavery but would have accepted it in it's entirety if it would have been a condition to end the Civil War and salvage the Union. It didn't end that way of course but is indicative of the political mindset of the period.