Yes, Thancred had learned from Natasha that magick was not exactly common on their star. It's a fact that had shocked him given that he'd met Stephen and Wanda first, but it's something he'll need to learn more about at some point. From Stephen, specifically, simply because he's curious.

"You're from the same place as Yennefer and Geralt, are you not?" Jaskier as well, but that is assumed, given that Geralt and Jaskier almost seem to come together as a package deal. Either way, Thancred's picked up on the fact that Istredd hails from that star as well. Referring to his world as a "sphere" being one of the dead giveaways. "There are quite a few of you here, yet there's no group of likeminded people fighting for your star's safety. Which invalidates your theory a bit."

He can't help but shoot Istredd a cheeky smirk then. He has tried to find some sort of pattern for who is summoned here and why, but he always manages to find some exception.