RJ wrongly stated that - like Semirhage - Graendal wasn't seen at the Cleansing, but that Moghedien just thought of her, and implied that he didn't want to show Graendal during the battle. However, other characters did see Graendal; she was obviously shown and described in Verin's PoV, even identified as one of the Forsaken.

Thus, one also can't compare it at all with RJ's quote about the Towers of Midnight just being mentioned by someone (Renna said in TGH, ch.40: "Deain's screams shook the Towers of Midnight, but of
course she, too, was a marath'damane, and marath'damane cannot be allowed to run free." ), contrary to some character actually seeing & describing the ToM on location. Graendal wasn't just "mentioned" by a character in the sense of this distinction.