“I, and everyone else, is aware of that letter.
The letter in fact reiterated it was about slavery.
That’s the odd thing about citing it, but it’s the only straw to grasp and it is easily misunderstood.”
I read the letter as giving absolute primacy to saving the Union, over the issue of slavery or anything else. A decision theorist would say that Lincoln’s preferences were lexicographic in: (preservation of the union and all else).
Destroying slavery is clearly delineated as the means.