Most likely this is just another version of the main characters life - they did the same thing with Here is My Exclusive Indulge 1 and 2. Same characters but they made different storyline versions.
I just watched Train to Busan today. Watched Peninsula too - not as good as Train to Busan. I still have AIB 2 to watch and looking forward to Sweet Hime 2 and Uncanny Counter 2.
hae yi issed him and they starting dating and the last scene was them walking while holding hands
He got his GED and then started college - it shows his GED certificate on the wall while he is studying to enter college then it shows him and Hae-e talking about their studies at college - it appears they are in their 2nd year of college because they are all discussing GH failing to get in the 1st time but he succeeds the 2nd time - a year later so we have a 2 year time skip at the end.