Littlepip told Spike that she lied about how Twilight died. Twilight didn't die saving ponies; she died trapped in Maripony after a megaspell hit the facility, dragged away screaming to be absorbed by the mutated gestalt entity she unwittingly created, which would become The Goddess of the Unity, the dominant intelligence of the artificial Alicorns that would terrorize the post-war Equestrian Wastelands.