Of course she helped him because blizz is pushing a braindead narrative, that Derek exists in the first place is utterly moronic and what has sylvanas to do with anything? There are other forsaken npcs that would be better suited to lead the forsaken or lay out their future.
Calia is not forsaken, is not resurrected through necromancy, is not impeded as they are. She is an outsider, a bland human cardboard box