You are aware that when Lincoln wrote that letter, August 22, 1862, he had already presented a draft of the Emancipation Proclamation to his cabinet?
Are you accusing him of deliberate double dealing? Even so, August 22, is well after Irvin McDowell set out for Richmond with orders to stop Independence, not slavery.
You can't make "Abolition of Slavery" a reason for going to war, after the fact.