So let me get this straight.
If she was talking about ARTHAS - That would mean everything post her turning on Arthas is completely 100% her Choice. Meaning absolutely no reason for a redemption arc as she is truly Evil.
If she was talking about THE JAILER - that would mean everything she ever did was pointless to her progression as a Character the moment she became a Banshee which means Sylvanas as she has been known the entire time People have been playing WoW is completely void. And makes the Redemption arc a massive copout to try and Salvage a character long past their due removal as there would be no need for a redemption arc for a Character with zero Character growth.
Either way Sylvanas is a shit Character to try a redemption arc for.