Your right he did say that but people are treating Nakyum like he thought the whole it was the learned sir. I know he doesn't want to admit it but for that mintue he knew it was Seungho. But everyone thinks he didn't know it was him lol I read between the lines alcohol gets rid inhibitions.
No, Nakyum was calling Seungho seonbi-nim (선비님), the term he uses to call In-hun. Seungho got annoyed by this and told him to call him nari (나리), which means master and refers to himself, and it's what Nakyum calls Seungho when he's sober. So Nakyum starts doing that, thinking that In-hun just wants to be called master. The translations are not doing a very good job of making that clear though, so I understand the confusion.
I don't know Korean so i'm not 100% sure but in the new chapter when Seungho was having some flashbacks about their first time I think it should have said "Ngh, Ahh... my lord" and not "My learned sir". Seungho is only remembering the parts in which Na-kyum refered to him explicitly by his title (나리).