Well, it's not necessarily teasing, I think in this case he was just referring to Gyuvin the way (many) fans would!! It's not out of the ordinary to use the honorifics that the person you're talking to would use.

so i think just using honorifics at all slipped him up for a second there... but it's still my theory that it means he has someone else he does call unnie. (i think with matthew it was also entirely an accident tbh)

Hao does seem like a probable candidate but if that's the case I'm shocked he hasn't said it to him, lol. Side note this made me wonder, I don't know if the gender swapping of honorifics is something gay people like to do in China?? but regardless I do think Hao would enjoy the way it's a thing in Korea lol.