Either that or bc I think he was rejected by him when they were kids that byul forgot about or was an air head about and that caused the rift between them.
I think the rift* is because Siwon's family fell on hard times and Byul manifested as an Alpha. In both social and omegaverse norm, Siwon realized he had a crush on Byul and probably thought it was impossible to be with him so he distanced himself. It's why he still tried to ignore him when he enrolled back at school. it'd be why Byul doesn't understand why Siwon ghosted him either.
A kiss in my opinion is way too small of a plot reason as to why Siwon would ignore him all the way to the present too.
Yall I think Siwon got jealous because he thinks Byul is still with his ex and assumes he's asking him for kissing advice.