Just how brain-dead those MCs can be, not focusing at all on that "You know better than anyone how tough things were for me back then". He needed to ask "just WTF are you talking about", as it seems that there is a huge miscommunication between them, and he knows a sh*t about what happened 5 years ago. From his memories FMC just yeet'ed from his life and city, the end, yet now FMC roasts him hatefully and his reachion is "oh, ok".
This story reminds me of pornhwa "Life with Mia" in many ways. FMC also showed up after 5 years, all grown up and cheerful, which isn't true to how she really feels.She has a cancer, just like her older sister that was MC's childhood friend/fiancee, in her case it's operable but she was nihillistic about living. Her sister ghosted MC just like here because she didn't wanted traumatize him even more with her death, and Mia also wanted to do the same but got roasted by him to cure the cancer, happy end x2(it's a succesful harem. So this one might be a non-h variation of that plot, it makes a sense, or it's about debts but most likely not.