
I think he isn’t accusing him, Jun is just aware that junwoo is crushing on another guy and this fact alone is the reason why Jun hates taesan. Jun is also aware that his existence to junwoo wasn’t as deep and important as he had hoped for …
Taesan is just some kind of variable to Jun and thinking about it, taesan IS the center of junwoo’s attention (attention meaning as bullying but also JW crushing on him) which Jun is jealous for (and he knows). He feels greatly inferior to Taesan as well …

I think your explanation does well in rationalizing why Jun was jealous of someone who was getting bullied. It adds depth to his emotions and the Jun-Junwoo-Taesan-plotline.
I wasn’t expecting Jun’s relationship with Yohan to develop like this, but oh well… like you said, we’ll just have to wait and see to understand what’s going on in their heads haha
I can’t stand the main character. Who in their right mind accuses a victim of stealing a bully’s attention, and even resent them for it?