"I think that's a bridge we'll have to cross later on. There was a reason some people stayed in Thorne when the others left." Yennefer is who he means. She thought she could get more done here, that having a position of authority would potentially yield good results for all of them. She's cozied up to royalty before, it is what she was trained for. And Istredd can't leave her, that's the truth. It is about the Singularity but it is about Yennefer in equal amounts.

"What did you mean?" Of course Istredd assumes that if someone acts bemused about Lucifer it's because he is difficult. It is a defining characteristic about his roommate.