Context matters. Lincoln wasn't saying he approved of slavery, just that keeping the union together was his priority.
I would save the Union. I would save it the shortest way under the Constitution. The sooner the national authority can be restored; the nearer the Union will be "the Union as it was." If there be those who would not save the Union, unless they could at the same time save slavery, I do not agree with them. If there be those who would not save the Union unless they could at the same time destroy slavery, I do not agree with them. My paramount object in this struggle is to save the Union, and is not either to save or to destroy slavery. If I could save the Union without freeing any slave I would do it, and if I could save it by freeing all the slaves I would do it; and if I could save it by freeing some and leaving others alone I would also do that. What I do about slavery, and the colored race, I do because I believe it helps to save the Union; and what I forbear, I forbear because I do not believe it would help to save the Union. I shall do less whenever I shall believe what I am doing hurts the cause, and I shall do more whenever I shall believe doing more will help the cause. I shall try to correct errors when shown to be errors; and I shall adopt new views so fast as they shall appear to be true views.
Slavery is a dead issue today. We can freely say that slavery ought to have been abolished and no one would disagree with us. That was far from the situation 140 years ago.
Lincoln believed that slavery in the states was a question for state governments to deal with (Southern radicals agreed with him, but went much further in demanding federal protection of slavery and its expansion to the territories). He also believed that his oath of office committed him to preserve the Constitution and hence the union. Lincoln's letter to Greeley was about priorities, not an expression of a lack of interest in slavery.
About a month after writing these words, Lincoln issued the Preliminary Emancipation Proclamation, a sign that emancipation was certainly on Lincoln's mind when he wrote to Greeley.
And in the same letter he said that if he could keep the Union by freeing all the slaves, he would.