The same way any other session-based multiplayer game, like any Counter-Strike or DOTA or Fortnite are not an MMO. MMO has "massive" in the name, as in "massive amount of players that you can interact with in one game session", and usually implies public spaces that are persistent and can be visited by any player freely.
Although, I must admit, the distinction is much less clear now than it was, say, 10 years ago.