“I think his understanding of the situation was exactly correct.”
That’s true. The Lincoln of popular history and the actual one have some significant differences. Douglass had more of an interest in getting the history right than most did.
Douglass had met with Lincoln on three occasions at the White House and he harbored no illusions about Lincoln’s goal.
Our pop history version is that Lincoln waged the Civil War for the purpose of freeing the slaves. But Lincoln didn’t say that, and he repeatedly said that his war was “to Preserve the Union”.
He made this explicitly clear in his letter to Horace Greeley:
“As to the policy I “seem to be pursuing” as you say, I have not meant to leave any one in doubt.
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.”
https://www.abrahamlincolnonline.org/lincoln/speeches/greeley.htm