Online != MMO.

MMO = Massively Multiplayer Online.

When you say MMO, you are talking about a "massive" amount of people (100+) in the same persistent game world.

A game like Warframe is not "Massively Multiplayer" (or at least it wasn't until Plains of Eidolon came out and even then that's debatable) because usually you get 1-4 Tenno in one mission and everybody sits in a lobby while they're not in a mission. That's just an Online game, much like Diablo or Borderlands.

An MMO game is where you interact with a game world that hundreds of other players also occupy and you will randomly see these other players during the course of play.

Mobile games typically don't have that, they usually have lobbies and matchmaking from my understanding.