Did you watch the first part of the story?Yul knew that JBY aka Mudeok was naksu, so him knowing the current JBY…
Yeah I watched the first part but details a bit fuzzy already.
So it is something like Yul knew Mudeok = Naksu in part 1, but he didn’t know Mudeok was original JBY.
So now that he knows Mudeok was the original JBY thanks to So-I’s tipoff, he probably could link that the current JBY had something to do with Mudeok and infer that Naksu is somewhere inside current JBY?
K-Drama 
So it is something like Yul knew Mudeok = Naksu in part 1, but he didn’t know Mudeok was original JBY.
So now that he knows Mudeok was the original JBY thanks to So-I’s tipoff, he probably could link that the current JBY had something to do with Mudeok and infer that Naksu is somewhere inside current JBY?
It seems like she only knows that Mudeok = JBY hence she doesn’t believe the current JBY is JBY.
Is it because it would make people think that the current JBY is a “fake” and hence it’s not a good thing for Jinyowon?
2) And how does Yul knowing Mudeok was JBY make him realize that the current JBY is Naksu? Lol my brain hurts.