At first I thought that Nightmare Granfalloon was just another impossibility alternate world far from main canon. Until one line from the story hit me. "All she knew was that she had worked up the nerve to finally confess her sin to the others today, and there was an EXPLOSION from Sugar Cube Corner or something." The indication being that Pinkie would have gone nightmare if her Battle in the Center of the Mind had failed! If she hadn't made up with Pinkamena, she would have gone nightmare, and lead to the destruction of the cast!
- Nightmare Granfalloon's line (quoted below) indicates that Pinkie Pie's "Pinkie" personality was strong enough to destroy all of her other personalities by herself! Angry Pie seemed to have no idea what she almost unleashed, and is really lucky that Pinkie listened to Pinkamena and decided to accept/absorb the rest of her personalities instead of doing away with them.
Nightmare Granfalloon: Anger is dead-dead-dead! Pinkamena is dead-dead-dead! Bad foalhood memories are dead-dead-dead! Now there's only happy thoughts!
- What's even worse? Luna says Nightmares are created when one part of them consumes every other part and Word of God confirms that the Nightmare can be different depending on what part of them that is. If Angry Pie had won, she'd have done just that. If the fun loving, Fun Personified "Pinkie" personality destroying the others created this Granfalloon, just imagine what would've happened if the Ax-Crazy, murderous, Sociopathic Angry Pie had been the part that had consumed Pinkie Pie and gone Nightmare!