He means what he always means.
The South would have been allowed to secede. And slavery would have been permitted to continue there.
Let's not pretend it's anything else.
He said the same about Ukraine. Trump would negotiate in a way that allows Russia to hold onto the part of Ukraine it occupies.
For myself, I think the anti-slavery element of the US civil war was morally just, and from a constitutional perspective, any state ought to be permitted to secede if it votes to do so. So you have a contrast of types of principle in play.
In my view, Lincoln could not allow the South to secede given its continued use of slaves (even if that wasn't his argument).