I’m saying the north made slavery their battle call only after other factors motivated them.
There were three compromises proposed before-and-in-the-early Civil War by which slavery would be forever enshrined in the US Constitution, in exchange for the South refusing secession. All three were rejected by the North.
Don't confuse the offer of isolation and a 'slow death' of slavery, as was a common abolitionist position of the time, with being neutral on slavery. The war was always about the fate of slavery; the position of the North only changed from "We will isolate slavery to ensure it dies" to "We will ban slavery to ensure it dies." Prominent voices in the North repeatedly expressed this view, and prominent voices in the South expressed understanding and horror of that view - with the South recognizing that slavery could not survive isolated in the US South, and had to expand to survive.
The proof they didn’t give two shits about black folk were the jim crow laws they allowed the southern states to adopt during reconstruction.
After the end of Reconstruction, when the Northern-supported Republicans lost control of the Southern states due to the Federal government pulling out, with the Feds pulling out due to a corrupt bargain that resulted from a disputed presidential election.
Also, SCOTUS struck down Federal anti-segregation laws during Reconstruction.