I’ll try to point out the key details.. which sucks bc there’s a lot more I wanted to say. The grandma was only holding the pillow to Sangwoos head until he stopped yelling, as soon as she held the pillow to his head she said he had stopped right away. And who knows if she did actually do it.. she seemed confused and senile. The hospital didn’t notify the police right away when Sangwoo died, their obligated to let them know.. the hospital also had two days to do so. The man in the wheelchair in the elevator had the same burns Sangwoo did and that scene was empathized. Lastly the receptionist changed her reply after finding out that police guy was a cop.
My guess is because the hospital was under so much pressure from the public, they wanted to pretended Sangwoo was dead so they would go away.

There’s a lot of plot holes and things in the story to suggest Sangwoo didn’t die.
I tried explaining but mangago didn’t post my comment.