That the opportunity to listen to Demandred & Mesaana's plans was a reward for Graendal, because the "Nae'blis has been chosen, but there will be other positions of high glory in the Great Lord's reign. Some much higher than others. Today was a taste of the privileges you might enjoy."

Moridin, who seems actually convinced that the DO will destroy everything, if he is freed, is just giving Graendal false promises here, IMO.