So, prepare for nuance. There is the slightest bit of truth in what they're saying. Lincoln did not initially make the war about slavery. Yes, the south 100% did leave over slavery, but originally the war was just about getting the states back together. It still feels incredibly disingenuous to say "the war wasn't about slavery" because of that though. For one side leaving it was, it just wasn't about slavery to the other side yet. I'd have to see the context of this comment but I feel hard pressed to imagine it as anything other than Lost Cause propaganda.
"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."
-Abraham Lincoln
Honestly brings up the question, what is the big point of "the union" if he doesn't even care about human rights. What's so groundbreaking about the U.S. if it's "democracy" with a big % of the population denied all rights? You'd think that'd be the highest priority.
He was definitely against slavery and that quote was in response to somebody else who had called him out for not freeing the slaves right away (https://www.loc.gov/resource/mal.4233400/?st=text). He actually wrote that line having already written up a preliminary version of the Emancipation Proclamation, so he knew he was going to be freeing the slaves when he wrote that.
I'm not a historian, but I think he may have been trying to obfuscate what he was doing, like he didn't want to come right out and say that's what he was doing. I think if at the time people in the North thought that the war was being fought over slavery and only slavery, they maybe wouldn't have supported it and maybe would have wanted the North to back down. Even if Northerners didn't use slavery themselves, it's not like there wasn't still racism all over, they wouldn't have been as willing to sacrifice their sons to free black slaves. Again, not a historian, but that's what I'm assuming was why he penned that.