For those confused, Daejun wants to show that Sihyeon (Hajin) is not behaving like Og Sihyeon. Before, he touched Sihyeon's (Hajin) lips and got bitten in return. Someone suffering from mysophobia would let someone touch him with bare hands and, even more, bite another person's finger in such a relaxed manner. I think Sihyeon (Hajin) is shocked because he didn't know Og Sihyeon's condition was that bad.