I think the whole point is that he had internalized homophobia since he didn’t realize he was gay yet. Jungpil was always someone who just expressed what he thought without much care for what others thought of him. It might be a good idea to go reread the first chapter or two of the other story and you’ll understand it better.
Also remember that the other story is like 2-3 years in the future so he has probably grown a bit thanks to when current mc lashed out at him. Remember he is still in his early 20s so the brain ain’t quite fully developed for him yet.

i read the other book, and why is Jungpil a bad sunbae here