Aptorian, on Apr 14 2009, 11:10 AM, said:

p. 846 in BH
Apsalar meets up with ST:
Quote
Shadowthrone made a whispering sound, like sighing, and one almost formless hand emerged from the folds of his ghostly cloak, gripping the silver head of the cane, that tapped once on the cobbles. "I watched, my dear. Your Shadow Dance. From the foot of the Rampart Way and onward, I was witness."
She said nothing.
Shadowthrone resumed. "Not even Cotillion. Not even Cotillion."
Still, Apsalar did not speak.
The god suddenly giggled. "Too many bad judgements , the poor woman. As we feared." A pause, then another giggle. "Tonight, the Clawmaster, and three hundred and seven claws - all by your hands, dear lass. I still... disbelieve. No matter. She's on her own, now. Too bad for her...]
bla bla the scene ends.
Shadowthrone nevers says "Cotillion could not have done that", what he is expressing is disbelief at Apsalar, whom they considered just a pawn, actually managed to kill not just the Clawmaster but 307 other claws.
Apsalar is a flawed carbon copy of Cotillion. She may have grown during the 2-4 years that pass between the ending of GOTM and BH, but to actually suggest that Cotillion could not have done what she did is silly. What ever Apsalar can do, Cotillion can do, backed up by about a hundred years of experience that Apsalar does not command.
If you want to translate Shadowthrones meaning, he most likely meant that he's never seen Cotillion do anything like that.
Never mind the fact that Cotillion destroyed the Edur fleet outside Drift Avalii, easily killing as many Edur as the number of Claws killed in BH.
EDIT: Hmm, that reply seems to be twenty minutes late

I took it simply as not even Cotillion expected her to accomplish something like that. Despite being possessed, they both probably had doubts about her.
Not anymore.