There is an immediate distinction made between Istredd and Yennefer as opposed to Geralt. Perhaps Thancred should have foreseen this. Given that there is clearly some history and tension between Yennefer and Geralt, it stands to reason that Istredd would be on one side of that or the other. And he seems to have taken Yennefer's side, for better or worse.

Given that Thancred is fond of both Yennefer and Geralt for different reasons and doesn't know the specifics of their awkward distance, it's been an odd thing to navigate. It occurs to him that he could ask Istredd, but he would rather hear it from one of them.

As for Ciri, he's aware of her, but hasn't met her, so she doesn't as immediately come to mind.

Either way, it appears that Istredd is talking around something, and Thancred can gather that everyone from their world is cagey in a different way. He raises an eyebrow, but doesn't press on it.

His mouth twitches to one side with what Istredd says next. "Surely it isn't simply an anomaly that there are so many us from the same star, or sphere, or what-have-you. I do think they can miss the mark, of course. It makes little sense for me to be summoned to Thorne of all places, for instance. However, I can only imagine they are at least narrowing it down to specific realms."