I think you are right—I was ending the story too soon (with him) and it still could have been a failure had they been shot. She couldn’t finish the song earlier because, as she says, “You weren’t there.” Now she’s finishing it, which means he’s “there”, and it buttons up whether or not they are zombies.
It doesn’t tie to the successful resolution of the story, but it ties Seok-woo as an MC to the OS. Without this earlier example, it would seem like a deus ex machina if it showed up later for the first time.