MMORPG is merely an acronym for a compound word "Massively multiplayer online role-playing game". The definition of Massively multiplayer online role-playing game is obviously far more complex than the sum of it's words.
ESO lacks interdependent group play, and is therefore not an MMORPG.
Even the dev's have strayed away from calling this game a Massively multiplayer online role-playing game.
For instance, Matt Firor in an interview was using the phrase Online RPG to describe the current direction of the game.