I have a few thoughts and theories on what the following quote could mean:



Perhaps there is a warding someone can place on themselves to deceive other females of the channeler's true one power strength. We see channelers completely eliminating the ability to sense in other women the ability to channel. So my first theory is that perhaps there is a way to make yourself look stronger than you really are. Maybe Cyndane deceived Graendal with her strength in the OP with some type of ward? I admit I'm guessing here, but it is possible.

Second, Cyndane is still strong enough to travel, on her own, because we see her do that when she goes to the cleansing. So there seems to be an immediate problem with Brandon's wording. NOBODY who can travel on their own is described as "very very weak." But then, maybe there is an explanation. In tGS we learn that Siuan's new self cannot open a gateway, but with a sa'angreal she can.

The only conclusion I can come up with is that Cyndane has an angreal or sa'angreal and uses it to travel, just as Siuan uses Vora's sa'angreal to gate to the rebel camp from the white tower in tGS. In fact, the last we saw Lanfear, we know for a fact that she and Moiraine were fighting over an angreal or sa'angreal as they fell through the gateway. Maybe Cyndane still has the angreal? If so, that would explain everything. Cyndane is weak, but with the angreal, she appears stronger than Graendal.

I've had my own personal theory for a long time that Mierin (Lanfear's name before she turned to the shadow) was not very strong originally. I have always guessed that at some point Lanfear went through the doorway where you get 3 gifts and one of those gifts she asked for was to be the most powerful channeler in the world. I think she got it, but now that she died that promise no longer is binding and she is back to her normal strength. Either that, or perhaps the DO somehow gave Lanfear extra strength for joining the shadow and in reward for boring the hole in the DO's prison.

Of course, the most likely thing is what everyone else has said, and that BS made a mistake. But he seemed very certain from how you're describing things. My theories up above could explain why Cyndane is weak. Maybe even combine one or more of my ideas above?