The Korea article was wrong. The director of the movie posted on his me2day (korean version of Twitter) and explained that Siwon will not be in the movie (it seem like he was supposed to. Probably his drama schedule made it impossible).
"Journalists are truly weird. If they just give one call to the company they would know everything, I don't know where they hear these things from strange places. I felt sorry for Siwon who was going to appear in my movie "Quiz King" but then couldn't. Siwon! Next time let's adjust time together. Now for your drama, fighting!"
Original source Jang Jin Official me2day
Translated by Alice (`Whiteuk별♯♪) @ sj-world.net: ♥