The "zombie musical" concept we see less of on the Blu-ray. but it's also a "playtime" sequence, just one that would have been Andy playing with his toys, perhaps in a way that would have led to Bo Peep's departure. Bo Peep is part of the playtime in the storyboards we see, so the scene would have taken place before she left. The scene also shows Bo fighting off the "zombie" toys with her shepherds crook, perhaps hinting toward the evolved Bo Peep we would see later in the film.