I believe that is a bit of an overstatement. Had Lee truly believed that slavery harmed the South then he would not have been so strongly opposed to taking any steps to end it. He was content to leave it in divine hands, regardless of how long it took. Those are not the sentiments of a man who viewed slavery as overly harmful.
Indeed his views are hardly different from the founding fathers like Jefferson and Washington. Both owned slaves and believed in the institution.
True. However nobody is making the claim that Jefferson or Washington was opposed to slavery, either.