No I mean that that quote is irrelevant towards you claim that the Emancipation Proclamation was an afterthought. The claim is made time and again that the Civil War wasn't fought over slavery, and from the Northern perspective that is very true. Lincoln's goal was preserving the Union and defeating the Southern rebellion, and the Emanciption Proclamation was one tool he used to achieve that.
Tool. Perfect description for the EP and Lincoln's purpose for it and exactly the part of history that we Southrons would like to have clarified.
Lincoln has been worshiped as the Great Emancipator when if fact he was just a union preservationist and a slick politician. The Emancipation Proclamation was just a slick tool.
If today's blacks could find a wormhole in time and travel back to see the real Lincoln in action, circa 1861-1863 (not the one they've been taught via the public school propagandists), they would have a much different opinion of their Great Emancipator.