And that is a fair question. Anyone who would trade slavery for dominance over others, certainly does not regard slavery as the priority.
Much of this article consists of his rather lame and entirely inaccurate attempt to say Lincoln was not particularly opposed to slavery.
Relative to ruling over the South, he wasn't. He says so many times. Between the two issues, slavery is less important to him than holding the Southern States.
Why are you quibbling over William's salient point?
Well... sure, people like to say that slavery was "America's original sin," but that is not a correct analogy.
In fact, slavery was a precondition:
Most Southern colonies in 1776, or again in 1787, would not have joined the United States if it necessitated abolishing slavery.
Without Northern acceptance of slavery, in 1787 and until 1860, there would have been no United States.
Everybody in those days understood this, and that is why, until Republicans came along in the mid-1850s there had never been and American anti-slavery party.
And those first Republicans then, just as many Republicans today, were the weakest of milk-toast anti-slavery politicians.
No major Republican publically called for abolishing slavery in the South.
All Republicans (including Lincoln) said they wanted was to stop slavery from expanding into western territories which didn't want it, or into their own Northern states via the Supreme Court's Dred-Scott decision.
But that was just enough:
Bottom line: slavery was as important to the North as it was to the South, for equal and opposite reasons.
But Lincoln's first concern was to defeat the rebellion which had declared war on the United States.
In that mission, he saw the Emancipation Proclamation as the military equivalent of several US Army corps.
In Lincoln's mind, as we would say it today, it was a "win-win".