The context of the statement I think makes her meaning clear at least. Lincoln had to make different arguments to different people in order to get the 13th amendment passed, arguments which might differ from his public position.

My goodness, Lincoln is such a perfect example of what she's talking about. His public position for the reason to fight the Confederacy was to save the Union, not to end slavery. But Lincoln made his abolitionist ideals clear in the Lincoln-Douglas debates. That's why the South was furious when Lincoln was elected, because they feared he would end slavery. Lincoln had to take the position that he was only fighting to preserve the Union, and that "if he could do so without freeing a single slave, he would", because he would lose the slaveholding states that didn't want to leave the Union. But, was his private position all along that he wanted to free the slaves, and might he have privately made that argument to the New England states to win their support to fight the Confederacy?