Abrogail is no longer entirely sure she's safe to say that it's incredibly implausible because Abrogail is no longer quite certain it's true. Keltham is right, in a way, there's - a sense in which Abrogail can give Sevar more powerful experiences, right now, than Keltham can give her. Abrogail was planning to do the most powerful version of that to Sevar that she could manage and leave her unshattered afterwards, well into the break-and-remake zone of torture. Things like that have been known to ever affect a submissive's feelings.
But if - that's all a 'trope' - is she able to just -
"Suppose that 'tropes' do have power here, and we try to just - not test it at all, walk away," Isidre says cautiously. "Does the 'trope' fight back, punish us for trying to defy it, I realize you're going to say 'no' but I'm wondering what the answer is instead."