My guess is it isnt behind him (obv), but thats what he is telling himself and pretending infront of Mokyeon as well to not show „weakness“. He probably didnt even process Mokyeons confession emotionally, since hes so (emotionally) exhausted and doesnt have the capacity to deal with it.
So thats why he was able to respond so coldly. He can also just tell himself its another way for Mokyeon to torment him.

Nanjo is cooooold, i love it. But i'm a little disappointed since i wanted the moment where he found out about Sunwha to be more dramatic. Like him catching Sunwha in the act of shit talking him or smth.
And he reacted so calmly as well. Maybe he found out before and just kept on going? Or subcounsciously he knew but didnt want it to be true. And thats why it didnt come as a shock?
Or it came as a shock and thats why hes acting like that?
Or he already left Sunwha and the whole ordeal behind him? But that would be really fast, since a few hours early he was still tryna ring Sunwha up. Hmmm
Next chapter better be his perspective.