Did you forget this line of the letter?
"I have here stated my purpose according to my view of official duty; and I intend no modification of my oft-expressed personal wish that all men every where could be free."
"I have here stated my purpose according to my view of official duty; and I intend no modification of my oft-expressed personal wish that all men every where could be free."
As it has no bearing on his intentions regarding the Independence of the Southern states, it wasn't forgotten, it was omitted deliberately. It is irrelevant to the point.
Again, the continued existence of slavery was Negotiable, but Independence was not.
Lincoln would let them own slaves, but he would not let them go.