Responses
I replied to your reasons under the other topic, if you haven't seen yet. TLDR: There isn't anything implying that Sihoo's mother told Sihoo that Myunghan's mother came over begging. They haven't seen each other since childhood, it was senior year of high school, it makes sense that Sihoo wouldn't be invested in Myunghan as they're essentially strangers by then. Think back to your own dropped friends.

There was a really bad transition where suddenly they're all in college now. The Sihoo Myunghan had been seeing was an illusion from his trauma of watching real Sihoo fall from a balcony when they were kids. Hyung (Seungchan) is a childhood friend Myunghan made after Sihoo's mom stopped letting Sihoo come over because she blamed Myunghan's mom for the balcony accident. Time had passed with the 3 of them not talking to each other while Myunghan was imagining Sihoo with him all the time. By the time Myunghan and Hyung meet again it's high school senior year, that's why it seems like they barely know each other again. Flash forward, Hyung confronts Myunghan about his hallucinations and then takes him to meet the real Sihoo, now grown up. It looks like this is going to be the end of the story and in this scene it doesn't really look like Sihoo is at all interested in Myunghan other than to say hi as they used to know one another, but then it looks like the author decided to continue the story into college as, next thing we know, they're randomly all buddy buddy with Seungchan and Sihoo seemingly not liking each other. The implication is that they have a rivalry for Myunghan. Though there was no buildup, we're just thrown into it. And here we are.