So there is one minor problem with the articles.

While it does accurately state that the slaves, spread over several properties not just Arlington, were to be freed by Mary's father's Will. It would only be after all his outstanding debts had been paid off and sufficient money placed in trust for his daughters. His debts and the decrepit state that he left Arlington in though ensured really that it never was complete, and it fell to Lee as the person charged with executing it to essentially not just take leave from the Army but do so for over 2 years, hence why he was there for the John Brown raid.

As for the larger question it is harder to say. And really related to any person's larger views on slavery to begin with. Lee did try to be a good Master, but at times treated them just like his soldiers and was thus often let down, frustrated, and grew cynical. But at the same time he could be affectionate to them. The slave who had served as the coachmen to his ailing mother, when he too grew old and infirm was freed by Lee and sent on Lee's dime to health spa's and we have letters expressing deep sadness at his passing and the last link to his childhood being gone.

An excellent book I would recommend is: Crucible of Command: Ulysses S. Grant and Robert E. Lee--The War They Fought, the Peace They Forged by William C. Davis