The term MMO is about the game/server architecture more than the size of the game. MMOs are online games with a single (or sharded) persistent gamestate. That's it.

Most shooters have rounds that restart, breaking the persistence quality of the game. Other games like Minecraft emphasize individual / private servers and break the "single gamestate" proposition. The "Massively" word refers to that single gamestate that many users can interact with, not how many do actually play.

To me, The "play offline" option is more against the MMO definition than the actual number of players.