Luke's understanding flows from Yoda, who told Luke the story while on Dagobah, while training Luke to kill Vader. So... In that context, Anakin did fail to balance the Force. In the context that Anakin then redeems himself and kills Palpatine, he balanced out Mortis, and then balanced out the greater Force. Luke maybe needs to think it further through, but I imagine he'll come to the conclusion himself. Denning might not let him, though.