I mean, it was already looking that way and now we know for sure with chapter 18. Since Ideum met the brother (Jiseong) at the swimming practice and didn't recognize him.
I haven’t read the raw haha I just had this speculation since a few chapters back
First off, sorry for the spoiler (just read the new ch in eng on lezhin)!
The author could always find a way to pull off a "it'd been a long time since he went to school with his crush and they hadn't interacted much so they didn't recognize each other and Ideum's memories are blurry from all the trauma" so who knows in the end. xD
But for the time being I think your opinion is correct.
My unpopular opinion is that I don’t think high school crush is the brother
High school crush has black hair and in that one shot where he was walking away from Ideum with his family there was no younger male sibling. There was also no confirmation yet that the fleeting high school crush of Ideum was a swimmer
Andddd in that panel shot of jaeyi’s brother from the back he has light hair
I don’t think they’re the same people. Maybe black hair high school crush looks like Jaeyi bc that’s just Ideum’s type