why can't he just confront and ASK.

He did but the timing was so ridiculous, they both misunderstood each other. Muyeong was like “a secret affair between someone from the publisher and the author. That’s it, right?” And Seonho went all gung-ho and agreed, “WOW HOW DID U KNOW” plus the name of the editor is close to his name so he thought Seonho wasn’t murmuring her name instead of his.