and Egwene doesn't say "vastly stronger than I am" ... she gets attacked by a mental TAR power and later comments on it. She makes no comparison to her own level of strength and never gives Mesaana's OP strength a second thought when they are actually facing each other later in the scene. If it was of concern it would have been in her thoughts. The entire scene is all about Egwene realizing that she needs to focus less on the OP in TAR and more on controlling the world through her Talent. Her confrontations with Mesaana in TAR are examples of this. In the first one she panics and flees because she is unprepared for this type of attack, she thinks "wow Mesaana is strong", this is in relation to TAR. In the next confrontation Egwene realizes her power is in her strength of Will and defeats Mesaana, all without the two ever touching the Source. It's part of Egwene's growth as a person. We've already seen that she is something of an OP prodigy.