The major difference between POW camps up North and Andersonville was intent. The Confederacy had diffulty supplying it own army, its POW garrisons and thus, its prisoners. The withholding of fresh food, especially fruits and vegetables was a Union policy....as was dumping dead prisoners in the swamp outside Camp Douglas. It was easy, they avoided taking accurate roles. On a positive note, it made escapes easier.
I believe George was making reference to President Davis' repeated offers to exchange Andersonville prisoners. Lincoln refused them all, even when Davis pleaded with him to accept the Union prisoners without conditions. The plan was to deliver them to the Virginia penninsula. They were more important to Lincoln as fodder for propaganda than human beings deserving rescue.