Mor Dhona is an entirelly seperate region, hence Revenant's Toll's aetheryte is listed in it's own section. It's on the very bottom though because it's an 'independent' aetheryte not claimed or controlled by any of the citystates as the region is not part of any city (even Ishgard makes no claim on it), so it wasn't grouped with them. It's not part of Coerthas politically or geographically.

Originally it was the only independent aetheryte in the game, but now we have a lot more independent aetherytes which have all been added to that 'others' list with successive expansions, thus forcing Mor Dhona to the very bottom as new citystates with their own region groupings were added like Ala Mhigo/Gyr Abania or Othard/The Far East). It's just pragmatism.