That does not mean they cannot blend in and bethesda just rectonned sybille to be a volk vampire. To me it looks like the vampires of the court don't normally leave the castle often.
She was not retconned, she was always a volkihar. Vampires do not always know about other vampires, so the priests info could be anything from simple rumors to only certain volkihar vampires recorded.
The court, sybille, hert, hern and alva all live comfy, the other vampires (the majority as well) live in caves or "underground" so yeah, no lore is getting broken except for the feeding thing.