I always considered “MMO” as a synonym for “multiplayer”. Anyway, if you take the word “massively” into account, you could also treat is as “massive number of players you could POTENTIALLY reach by joining one of the rooms” (word “potentially” as opposed to “actually reaching at any given moment”). In actual MMORPGs it’s not like you can actually interact with all the people online at the same time, you still are limited by geometry and game logic of the world at least, like no one will ever gather in a single town because people are doing other stuff, or are from enemy factions, etc. And in many MMORPGs there is instancing going on, so even players in mob farming locations can be assigned to different instances and not see each other because of reasons.