I agree the letter really isn't news. We do need a source to show "his" slaves were whipped for disobedience, however. They we're his father-in-laws, not his, if we're talking of the same bunch.
It's such a small point but an oft-repeated irritant; employed in the attempt to try and soften that the primary tenant of Secession and the Confederacy was slavery. It goes like this: "because Lee didn't own any slaves it means that for him the war wasn't about slavery." Edited.