I think you could definitely interpret it as a combination of everything moving at lightning speed for treating something likely terminal, and him probably thinking he wasn't going to survive.
Well... If you want pov on this,
If I remember correctly Theo was in coma for 2 years after the the surgery and years of not able to do anything so I'm pretty sure in that time no one would remember to give him the documents of a girl she met and stayed briefly, also I'm sure Theo didn't tell his family in case he died and they tell Hyeonji.
Also it's like what 40 years in the past in rather remote korean village, so... If the present let's say 2025, it would be in the 1985... Yeah, no you won't find even find a phone let alone camera to take pictures with.
Rereading this for the nth time. I've only now just thought about Haebeom's dad seriously, and what I would do if I were in his shoes. I understand leaving Hyeonji because he didn't want to burden her, I'd probably do the same. But what I don't understand is why he didn't seem to have done anything to help him remember Hyeonji? He knew going in the operation that there was a high chance he'll forget. Why not have a video recording of yourself describing her and the life you had? A photograph of you two? Heck, even a journal about her will do. Even just a little bit of trinket that's important to you or represents the life you had together. They lived together for a year, it's impossible they don't have any.
Maybe it's just me as a person, but I am selfish enough to want to know about the person I love even after leaving them. I know myself enough to want to know if I were dating someone, and was serious enough to live with them for a year.
I'd love to hear other reader's perspective on this.