When I argue this topic, I point out that very thing to the people who think it was all about slavery. It's like water off of a duck's back. They won't absorb it and they won't acknowledge it.
Never quite explained is why slavery in the Union states was not terminated during the war and said termination did not happen until after the 13th Amendment. Supposedly, there were men from Union slave states fighting to free the slaves in the South.
I make this point often as well. I tell them that if the war was to free the slaves, they could have started in Maryland where their supply lines would have been much shorter.
People want to believe what they want to believe, and they will persist in believing it despite good evidence that they are wrong.
The only thing you can do is keep tossing things at them that don't make any sense from their world view in the hopes that they will finally see the dichotomy.
If this had occurred, was there a concern that Maryland and potentially other border states would have seceded and joined the Confederacy?