
the translation is really confusing so i don't blame you for not understanding lol.
basically, inseob (friend) was initially kyung's (top) type but only at first glance. he eventually became more enchanted by woojin (bottom).
kyung has a pretty nasty, manipulative personality and isn't straight-forward with his feelings so he tried to manipulate woojin to give up on inseob. that includes making him promise to never confess to inseob and also confessing to him, to get some sort of reaction out of woojin. he wasn't serious about it though as shown that kyung was more upset about woojin leaving for the military than showing any sort of actual reaction.
woojin, however, is just as dishonest about his feelings and just never really dismisses kyung's assumptions.
so kyung doesn't know that woojin returns his feelings. in his eyes, woojin never really gets over inseob. he thinks woojin is still pining for inseob after all this time and he thinks woojin would prefer literally everyone else over him - which is why he eventually snaps at one point.
it's basically one gigantic misunderstanding because woojin is too scared and kyung is too proud to talk lmao.
yall idk if my attention has gone to shit or if it's the translation quality, but i'm still super confused. uke liked the seme since school days, that's easy to get. did the seme initially like inseob and later (after uke finished his military service) started getting interested in the uke?? or did seme also have a crush on the uke when he was in school? if so, why tf did he confess his feelings to inseob???