You're correct. The explanation was the closest he could get to his soulmate (which was Hangyul) was the brother, because of the botched soulmate string, which was why he was so infatuated and mistaken with his feelings. The brother was also sure he was not serious and he never genuinely liked him, too. I think he started questinung things after that conversation.
He was always wanting to "fix", for the lack of a better term, that thread anyway, and now he's getting drawn by knowing he can change the course of things and save Hangyul, then the sexual dreams, them being isolated in a foreign country, and knowing that they are supposed to be soulmates. But you can blame the dreams mostly, I also feel weird around people I dreamed about hahaha.
Tbh, I was so happy that the MC finally gave a reliable big brother figure.. only for him fell in love with the MC
Don't get me wrong, I probably read with my eyes closed... But wasn't the guy like always playfully flirting with the MC bro, the glasses one?
Now hes so hung up over the thread