You know, I was thinking the same thing. So i get ya ma dude!
I get it but it's not accurate. Many times Songwoo has tried to tell Sangjin his feelings but he's been interrupted. The falling flower pot happened when he tried to change the outcome too? Why do people either not remember that or think a falling flower pot isn't deadly? And I don't see Songwoo doing what you're saying at all.
Lol I was on 6th chapter when I replied that..(￣∇￣") And as k have said Songwoo was testing the things in the diary all along.. and that pot was deadly.. But I don't think if it was cause of the diary .. Now cause of all the misfortunes occurred he will think he didn't believe in it so they occurred that way...
It might be a spoiler cause there are some people who think giving theory is also kinda like a spoiler..
This event or story written in the book is kind of like a premonition for the future of Sangjin and Songwoo.. See if the diary is kinda like giving him a warning "If you don't make a move this will be the outcome"...
Cause unconsciously Songwoo is inviting him now and asking him about his feeling even thou in a friendly way it's a great progress ..Maybe it's to check the truth of the story in the book but if he never had seen the diary he would have hidden his feelings and never even jokingly say or confess..
Ughhh! Hope somebody gets my theory lol..ლ(´ڡ`ლ)