Yes, so? He didn’t want to use slaves as soldiers.
He also said that he believed the relationship between slave and master was the best possible situation for blacks and whites in the confederacy. That doesn't sound like someone who believed in the imminent demise of slavery to me. And actually the gist of the letter was that Lee did believe that blacks should be used as soldiers because of the dire straits that the confederacy found itself in. And it is interesting that when the confederate congress finally approved the use of blacks, free and slave, in combat units, it could not bring itself to mandate that those slaves conscripted as soldiers be free after their service.