James McPherson is a historian who always seems to take an anti Southern position. I would not trust anything he says or even quotes.
I would even suspect as inaccurate if he had a letter supposedly in the hand of Robert E. Lee. Much original research is done, even if from an honest detachment then found out to be researching phone documents which have been said to be originals.
Try reading article one section nine of the confederate constitution
By the way I didn’t claim he had a letter from Lee. There are plenty of original documents at the confederate civil war museum. Go see them