The reason is because since he knew what he looked like and now that it's was confirmed that he went to uni he could look through photos and find his face and confirm he was a boy
I think it's more along the lines of they know where he goes to school now they know where to find him or maybe stalk him in his school or pry into his school records??
They had a set story for his female alter. I'm guessing t university wasn't in the backstory so saying he went to that uni blew the cover.
Gives more leads if they’re trying to find out if he’s a guy
Ah thanks everybody for explaining
I don't get why they are making a big deal out of him telling the brother he studies at Tokyo uni how is that exposing he is a boy is?. Are there no girls at the uni? Plus the brother already knew