Responses

A plot device for drama ╮( ̄▽ ̄)╭

They were kids. Siwon could have misread or mispronounced Jinha's name and have been calling him by the wrong first name all his life without realizing it. I have seen in several manga where someone misreads someone else's name, or they give them a nickname because they can't pronounce the real name (my best friend Krystal became Khistle because that's how her nieces and nephews pronounced her name when they were little)
Seriously, they just need to talk to each other face to face and catch up with what happened. ( ̄へ ̄)