So what does "massive" means when regular multiplayer games are just as big, if not bigger?
Elite Dangerous
Persistent world? - check
Supports large number of players online in that persistent world? - check
Players can play and interact with each other within that persistent world? - check
Please...
The number of players is by no means large, many multiplayer games have higher player counts than that. I've seen Flash games with more players, and there's nothing "massive" about it. As for playing and interacting, oh well, players struggle to even talk together, and the only worthwhile group activity is to shoot each other: the online is pretty much worthless, and while it is online, it's not really multiplayer, it's closer to people playing a singleplayer game in the same universe.
As for being persistent, it's never really been part of the definition of a MMO. And then, people tend to be confused about its meaning: it means things keep happening even when you're not playing, and these days, it's the case for pretty much every multiplayer game, considering persistence starts as soon as there are three players who can join and leave the game at any time and pick up right where they left. The game sure has plenty persistent elements, but their depth is lower than that of your average FPS.
What you define isn't a "MMO", only a mere multiplayer game, and that's what Elite: Dangerous is. Unless all online games are MMOs, after all, many caught up with MMOs of the past, but then that's what a MMO strives to be: bigger than common online games, and with mechanics fitting of that massiveness, but I'm seeing neither.