I respectfully disagree with your characterization of Lincoln and the objectives of the Civil War. I would assert that Lincoln fought the war motivated by both objectives: preserving the Union AND abolishing slavery.

I am very familiar with the quote you cite. In the context of his life, I think of that quote as trying to motivate support for the war effort—he say people more interested in preserving the Union and not so much enthusiasm for abolishing slavery. This quote was, in my view, more of a pep talk and not a policy statement.

Think about it. If Lincoln ONLY was motivated to preserve the Union, he could have simply legalized slavery. There would have been no war.