She thought she’d be playing a character named Lindsay, not the legendary Talia al Ghul.
Interestingly, the Arrow team sometimes even goes to great lengths to keep secrets from actors about the characters they’ll be playing.
Lexa Doig went on to tell me that she didn’t know she would be playing Talia al Ghul until she met with the costume designer the day before she started working on the show.
In the case of the Prometheus reveal, I’d argue that it worked well.