General Donald Piatt travelled with Lincoln when he was making his campaign speeches and knew him intimately.
There may not have been any such person. Abram Sanders Piatt was a general. His brother Donn Piatt was a colonel. He got into trouble enlisting slaves (apparently freedmen) before it became government policy and became something of a hot potato politically. There was political pressure for and against his promotion to Brigadier General. Lincoln felt Piatt was too controversial and vetoed the promotion. Consequently Piatt had a grudge against Lincoln. After the war he served in the Ohio state legislature, where his radicalism again made him enemies. Piatt became a Washington journalist, with the cynicism and nose for scandal of the profession. All in all, he's not a particularly creditable witness.
One of the themse of Lincoln's story is that he was not appreciated during his life. Whatshisname is going on about nothing special.
Walt