I don’t get it

In Detective Conan, Hagiwara died from trying to disarm a bomb. 4 years later, Jinpei dies the same way. They’re both side characters in the show and most of the screen time they had are just flashbacks. In this doujinshi it puts them as a couple in high school and skips to them already being police officers (Which is why Jinpei is still alive and he’s “hearing” Hagiwara as if he was still there.

Thank you so much