@Isekaijin I don't think that he's retconning that much. I mean, the evil spirit is a thing (Lemur appeared in the original manga), and manipulating someone into an overgrown ambition has happened in the series before (Poseidon was manipulated by Kanon, Pandora was manipulated by both Hypnos and Thanatos) so this is not that far fetched.
Still, I think that this arc was more or less unnecessary. Saga is stablished by this point in all media as good, if sometimes ruthless. There's no need to paint him as even more tragic.