I think it’s for multiple reasons, not just the collection.
He's already caught feelings for Jin, he's just reluctant to get in a relationship with him cause he worries that Jin's feelings are either fleeting or aren't strong enough. Seeing how long Jin has liked or at least admired Hajoon and how much he likes him (enough to be have a bit of an obsession) that might be why Hajoon seems to give in little by little, not just because Jin had the same magazine collection.
My POV saw that by seeing these magazine he was in charge of for years in his room, he just saw how jin is actually very honest on everything he said about liking and kept searching for him. Beside the fact that he enjoys himself when he is with jin, seeing that must have move his heart lol. I would too tbh, if the person is as honest and sincere, fun and charming like Jin.
I mean, Jin clearly appreciate him as a person and even appreciate his job (his hard work). He then became a devoted fan that even applies for a small magazine when he can actually achive higher just for him. Who can redist that?? Especially hajoon who was never been treated that way-I mean being appreciated in secret even when he doesn't even do anything for jin for years- by others (or his ex sex friend jaeha who only seek pleasure, and anyone would do, not seeing hajun for himself)
Ok na cause what? He changed his mind cause he saw his magazine collection? Am I missing something, people don’t usually catch feelings from that shit, it seems odd.