Actually, it's not bad writting, just that you're a bit slow with the way it's written. Not being mean by the way.
Basically Sowoon submitted his real diary by accident. All stuff written in there were stuff that already happened, and Mr. Heo was basically imagining everything CONCERNING Sowoon's diary. The timeline might be off for you, because it switches to Mr. Heo having his daily struggles at school (LOL) and then the same Heo picturing the diary of Sowoon.
by accident? girl it was made explicitly clear that the diary IS the work he wanted to send in (with the full version being available for reading online, like what the TA found) . I'm saying the writing's off BECAUSE it wasn't made clear that it's switching povs and timeline, which the author should be aware of when making stories with multiple of them stringed tgt, that's basics of fiction writing
I suggest to reread the 9th chapter again. It clearly says that Sowoon turned it in by accident.
I'll type it for you:
Mr Heo: "You turned in the wrong manuscript, right?"
[...]
Sowoon: "So that's what happened. I'm such an idiot. No one else would've made that mistake. I kept on worrying that someone might have read it."
Now, if that doesn't make it clear for you that he turned his diary/novel by accident, I don't know what to say bro. Maybe fix your reading comprehension
if sowoon is actively writing like as the days come, why the fuck is the work handed like as a one time shit ykwim like he gave the TA the work already and the story is unfolding as it goes and it's not like he gave him like a daily update or smth so is the story just a prediction of the days that come leading to the other guy leaving? like imo there's so many like weird messy plot holes going on like a lot. hence why j feel like this is bad writing. it's a great concept for sure and the art is amazing but author should really re-examine their story flow again