HELLO???

This is translated from another language and we does not necessarily include him. It could be here and the girl

If only OP!!! I wish!!! Heheh sadly… The original Korean line is “이렇게 많이 찍었었나.” Which doesn’t indicate any person specifically who took the photos. It’s just saying along the vibes of “this many pictures were taken huh?” So the “we” in the official translation is probably the general we as in the family or himself or grandpa etc.

Sadly it doesn’t mean the same as “WE took”

If only…. Sigh…

+1 for this.
"We", "our", "us" can be used talking about the speaker + the other person/people.. not necessarily the one he's talking to. It could also be granpa and Yoonsung only. Remember that there are photos in there taken by grand pa too. :)

so i was delulu all along lol haha... if only he had actually said "we"

A little spoiler if I get the translation from the novel right(honestly used google translate here so I can't say I'm 100% correct. Here's hoping that they would release an official english translation. I don't mind spending more for this cos it's really worth it)..
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Anyway here it goes, you're kinda right when you said "he knows".. at some point Yoonsung has an idea that it was Hakyung but not 100% proven. You know that thing that he was convinced that Hakyung is the other person given his reaction when they met again but still has yet to remember anything happened between them during those days. It's instilled in him that it's Hakyung all along no one can change that but their story is what he can't remember.

You’re not entirely wrong tho cuz at this point he sort of knows the truth so he could also be referring to HK :) it’s just not explicitly said that way. Hehe

I'm not totally surprised though, to be honest. It seems like recently he's not really set on necessarily "finding" his mystery person per se.. he's just decided it HAS to be hakyung and now he just needs to remember the exact memories. That, or until Hakyung lets it slip at some point haha