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.
Edit: didn't realize you were the author, asking a practical question about how to describe your project. Sorry about that. For that, I personally think you're fine. You're trying to tell people what it is, not be super technically accurate.
So if they weren’t the author you’d be tickety-boo with this smart-ass rude reply? Christ almighty.
On the contrary, if you're trying to convey to a mass audience that what you have built resembles something else, the best strategy is to reference that thing by its most familiar name.
Sorry if I came off as petty. Was just trying to answer honestly, and realized I didn't have the right context.