They didn't bring her back.
On the most basic level a backup is a copy created to replace the original, and the authors haven't established a reason why this would be different, or why anyone should think this is different; so lets call a spade a spade... she's just dead.
Because a 300 year old, projecting on a 20 year girl a life they never lived together and a person she may never become, would go so well. (That is sarcasm)