I would have thought that it was common knowledge at their school that he is Wine's big brother and also became a vtuber. Wine has also been quite clear she's not going anywhere without her big brother, so if she's watching her even a bit, she'd know that he'd be with her.
From this, I can only conclude that she's been living under a rock and has no interest in any other vtubers nor her genmates and now she's surprised to find someone from the very same library committee right there.
But with her reaction and the MC being the doofus he is, he's going to doxx her (well, say enough for people in the library committee to put the pieces together and figure out she's a vtuber, and then the rest of the school will hear about it).