Why do you suppose he made that statement. Here's a clue: if you look beyond the limited resources of mises and dilorenzo you can find the answer.
Probably those in the North whom Lincoln didn't see fit to set free with his Proclamation.
Incorrect.
If the war was over slavery, why didn't they unilaterally end it when and where they could?
Because they recognized that they needed a constitutional amendment to do that. The irony here is that if the slavers had kept their pants on it likely wouldn't have happened the way it did and certainly could have happened without bloodshed. Selfish, impetuous bastards.