Keltham snuggles quietly. It's actually just dawning on him that this is a lot of evidence that - why doesn't his System 1 believe it's a lot of evidence that masochists are real? Because his System 1 was expecting the Conspiracy to have faked this successfully, by the time he got around to actually checking it this much later? Clearly so, which is why his System 1 doesn't want to update and claims this is just what was already predicted.
Keltham can't back his brain on this! It didn't have to be that way, probabilistically speaking, there could've been some weird obstacle to Yaisa enjoying herself, Carissa could have said they needed to import a masochist from offsite instead of pointing to one of the existing students, Carissa could have claimed masochists were rarer, Carissa could have not suggested this particular evidence.
The Conspiracy world where masochists don't exist is now smaller and narrower, and has more of its probability mass concentrated into modes where the Conspiracy's capabilities are high enough that Yaisa doesn't really exist or was reprogrammed to actually be a masochist. Either way she's not a threatened innocent whom Keltham is hurting.
This is just like the thing where he forgot why he originally started to suspect a hidden cleric, that the original evidence was that the attack's timing suggested eyes on Ione but not on Keltham, and he worried that the hidden cleric was Carissa. There's the evolutionary logic that says masochists shouldn't particularly exist, sure, but probably a lot of the real reason his brain became worried was that Carissa wasn't able to go above low arousal with him. Her having just come off seven years of emergency response fighting demons is a plausible reason for her having difficulty relaxing. Fine, the Conspiracy had somewhat more ability to select Yaisa before bringing the twelve researcher-candidates to meet Keltham in the villa library, but if the Conspiracy can find masochists at all it means masochists exist, if the Conspiracy can make masochists they could have applied the same procedure to Carissa. If they can find or produce Yaisa, why have Carissa merely pretend to be the same thing and then get caught.
There are still stories where it's all fake and masochists don't exist but they're now less plausible and his brain needs to recognize that the flaming probability went down okay. Literally update at all, here. The probability of masochists being fake did not just go up, it did not just stay the same, therefore it went down.
If his System 1 wants to claim that it already mostly expected this result, then it should admit that it earlier narrowed down the set of possibilities to the Conspiracy either having the power to make Yaisas, in which case why not remake Carissa, or Yaisa was already real, in which case why have Carissa merely pretend to be the same way. Keltham is repeating himself here, yes, but he's repeating himself because if his System 1 already expected this result, it needs to respect the probable reason why that result would be expected, or else reveal that it secretly suspects some different story.
...his brain yields. A little. Grudgingly.
...Keltham's deliberative process will take it, he guesses.
In time, feeling a little awkward about it because Civilization has protocols for dates ending and Yaisa knows none of them, Keltham tells Yaisa that he'll see her tomorrow.