I'm assuming we're both using the term "MMO" as shorthand for "MMORPG", as in the traditional shared-world environment with character progression, teaming, quest-like plot driven objectives, consistently seeing the same people if you're playing on the same shard, etc.
If you're instead using 'MMO' to mean anything with lots of players, then we're talking past each other, so nevermind.
This page suggests that the multiplayer component of the engine is elastic supporting as many players as your willing to host (and can afford to host)
It doesn't say you can have all those players sharing a single, consistent world. The implication that it can't easily do that comes from the list of game types given as examples; all of which involve short, relatively small multiplayer sessions logically disconnected from each other except by a matchmaking system.
Millions of players, and not being an MMO?
Yes, that's what a MOBA is. League of Legends has millions of players, but you only ever see the ones you end up in a match with, which are usually those with a skill rating close to yours. There is no running around socializing with random people that you aren't already in a match with (or have on your friends list).
Some first person shooters have millions of players and are not MMOs. A few recent ones have vaguely MMO-ish aspects (Destiny), but are not a full MMO in the sense that most people expect when they hear the term.
Sandbox games (a la minecraft) have millions of players, but do not feature a shared world. Instead, multiplayer consists of one person hosting a world and others visiting it in a transient fashion.
Think its too early to be assuming the worst.
No one here as tried the engine yet, so why assume it cannot handle MMO development?
How is believing that it's designed for what the information page says it's designed for assuming the worst? I'm sure it'll be great for small studios wanting to jump on the MOBA bandwagon without a huge server investment. If it turns out to be cost effective, we could see some very interesting indie development as a result.
But why assume that it can handle MMO development when that's the one type of game that was excluded from the otherwise very specific list?