I didn't mean that quote as a contradiction to your statement. Indeed Lincoln had a personal wish that all be free [1]. My point was that slavery was an ancillary issue to both the start of succession and the war to end it. If what powered the south's livelihood was--let's say for the sake of argument--oil and the north was looking to ban oil as reprehensible and to make it a legal obligation to use only renewable energy then the outcome would have been identical.
Yes, Lincoln being personally for renewable and himself anti-oil would certainly have affect voting blocks respectively. But I maintain that it was still only a component of what started and sustained the war; the south saw the north as an existential threat to their mode of existence. By disallowing new regions statehood unless they outlawed oil the north was severely disrupting the previous balance of power.
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[1]: "I have here stated my purpose according to my view of official duty; and I intend no modification of my oft-expressed personal wish that all men every where could be free." -- from the same letter