It's possible for Graendal to be impressed with Cyndane even though Cyndane is now weaker than Semirhage. Graendal is meeting someone completely new, someone more than a regular Dark Friend channeler who is being brought to Forsaken meetings, that she's never heard of, and she is stronger than herself, when she knows she is about the 3rd or 4th strongest woman in the Power, ever. When you are used to only 2 people stronger than you, and one of them is presumed dead, and now a completely new one shows up, it is a shock. I don't think Graendal's reaction to Cyndane is any marker that Cyndane is still stronger than all other female channelers.