There are a couple of distinctions here: Habitat is graphical and not merely text-based. "MMO" means "massively multiplayer online [...]" but MUD1 and the minicomputer/Unix MUDs were not "massive" but limited to a few dozen players at a time. It is unclear how many players (registered or simultaneous) were supported by Habitat, but Wikipedia seems confident to call it "MMORPG" without numbers or a cited source.

https://en.wikipedia.org/wiki/Habitat_(video_game)

I was involved in the campaign to increase TinyMUD capacity from ~64 players to ~256 simultaneously connected players in the mid-1990s. But nobody was calling that "massive" even when it seemed like more than we ever had.

There are other criteria that may be considered for a "massive" adjective. Size of the world, and time to explore. It seems that Habitat was rather expensive by the hour! The MUDs in academic networks were populated by students with a lot of free time, and possibly unlimited access from their CS-related majors.