The burden of proof lies with the person making the claim. You can’t say “omg that never actually happened!” and then say someone has to find proof for YOUR crazy claim.
They turned their back on it, but that’s not what retcon is. It’s directly referenced multiple times throughout artifact questing. It’s the entire reason you get Felo’melorn.
You literally meat up with Aethas Sunreaver and he states “I assure you if I was able to join you in Dalaran, I would.”