To all intents and pusposes the population of Zwift is much smaller than that as far as the player goes, because Zwift only shows a maximum of 100 riders on screen IIRC.
But the reason I compare it more to a driving game than an actual MMORPG is that I feel that’s more waht Zwift boils down to. It doesn’t have many of the subsystems of an real MMO game. There are no “quests” for example, no competing factions, no overarching story, no customisabililty (e.g. player-driven in-game events or scenarios, beyond a group ride or race anyway), no “encounters” or NPCs, no sandbox elements, etc. etc.
Even compared to a game like EVE that also has no prescribed story, Zwift lacks MMO gaming features; there’s no free agency in where to ride for example. Fixed turning points, and a completely fixed path outside of that minimum of player agency. That’s more like racing round a virtual Snetterton than playing an actual MMO game.
It’s “massively multiplayer” insofar as thousands can be logged in at the same time, but there’s little in terms of interaction either. We ride through each other, or around each other automatically. The ability to give a “Ride on” or press a sound effect is about the extent of player interaction. There are some gamified features such as the prize wheel at the top of AdZ, but once you’ve got all the prizes that is no longer relevant. Particularly at the level cap.
Where it sits at the moment, the “game” part of Zwift is pretty minimal.
I’d love to see Zwift add more actual MMORPG type events in-game. Things that might require some sort of additional UI elements, thus more flexibility to enable us to tweak it to our preferences.
I dunno, maybe a quarry game where one rider gets a 2 minute head start before a chase pack starts hunting them. But unlike a “hare and hounds” race, the quarry has free agency to make turns, and the UI shows a “scent trail” for the chasers. Maybe the trail is hidden within 100m of a junction so you can’t see which way they went without making a turn and then riding that way for a while. The quarry’s UI would display his current points from the event and an indicator of how close the chasers are. Similarly the chaser UI shows the scent trail getting stronger as they close in.
Or, or, how about some sort of boss fight encounter, where the only way to defeat the boss is to combine the power output of all riders who choose to ride over to the event. They’d have to sprint in certain phases, and back off in others (e.g. when the boss has a damage reflection ability up). Everyone present in the boss kill gets an XP or Drop bonus in line with the effort they put in to defeating him (TSS-related or something).
Dunno, spitballing.
But right now Zwift is just get on a ride your bike. And that’s plenty for many people, of course.
I’d like more. 
And minigames. Like Devils, delivery challenges, orienteering challenges.
Then we need a far more customisable UI.