"Yeah, it was only in one of the books I read back then." Magnus is telling Lan Wangji this because he's in his telling-Lan-Wangji-(most)-things-again era, and because he knows Lan Wangji won't go blabbing about it to anyone else. He's probably going to tell Galahad for the same reason, after he decides if it's okay for Claudius to know -- Galahad will definitely tell Claudius after Magnus tells him. But that's it! "He probably has a title or something. I don't remember and I didn't ask." He couldn't find the book again when he went back to look for it the other day.