I think that is Starlight's alternate universe. She caused Equestria to turn into dust after destroying the only thing that keeps Equestria alive. Without the strong friendship of the mane six, Equestria would literally not exsist. That's why Starlight Glimmer was so suprised. She realized that she made that alternate universe and literally ruined Equestria by tearing their friendship apart. That just shows us and Starlight how strong the friendship of the mane six is. It may sound cheesy, but their friendship really is magic.