Of course it is, and that is the real point, secession and the war were not *only* or even primarily, about slavery. Lincoln only freed slaves in those states "in rebellion" and not in states such as Maryland. He's on record as saying if freeing the slaves would help preserve the union, he would free them, and if not freeing them would do so, then he wouldn't.
From Lincoln's perspective the war was about forcing the Southern states to remain in the Union, despite the wishes of the legitimately elected legislatures thereof. For the South, it was about defending a newly declared independence from what they saw as a Northern oligarchy.
Southern sympathizers can twist this issue any way they want but at the very core of the sectionalism that brought on the war was slavery. The overwhelming issues which divided the north from the south in the years preceding the Civil War were about expansion of slavery into the new territories. Without slavery there would have been no civil war. Southern apologists would love to paint this as some sort of effort by the South to cast off Northern dominance, which is unbelievably ironic considering the utter depravity of the kind of dominance the Southern slave holders kept over millions of blacks. Lincoln never made a secret of why he freed the slaves only in the States which had seceded, but there was also never any doubt after he emancipated those slaves that when the war was over slavery would be abolished.
OK, you've shown that the North didn't fight the war to end slavery. But can you show that the south didn't launch their rebellion to protect slavery?
And Lincoln insisted on including the 13th amendment in his 1864 platform, which DID free the slaves in all states. If a Confederate agent hadn't killed him, he'd have lived to see the amendment ratified.