Isn't it because boss isn't important in his life yet and it's just one of passing events in his life. 7 years is enough to forget a classmate what's more boss is a gangster which you wouldn't immediately remember or associate with a classmate unless they mention it. Boss never hinted at it.
But it wasn't just a fleeting moment. At least, not something someone could easily forget as if it didn't happen at all. Jejin and Boss Taekmu isn't just merely classmates. Jejin went after Boss Taekmu, even cooked for him in his house when he was sick. I don't think that's something that can easily forgotten even after a decade even for someone like me who has a memory of a gold fish. But idk. Maybe this manhwa has a different logic
I'm really concerned how author-nim will explain how and why Jejin can't remember Boss Taekmu. But however the reason is, I feel like it's all gonna go down with the sloppy amnesia idea. And if that's the case, this re-encounter trope just feels so... forced. Idk, but I'm still holding out and keeping my fill judgement until everything's revealed, so...