Sharding, and many other finer (game and architectural) design and implementation details. Few MMOs actually have millions of simultaneous players in the same instance of the world or boss or raid at the same time.

The MMO-lite I worked on was made to be as horizontally scalable as possible, and had extreme incentives built into the gameplay to push players towards actions and areas of the game universe that would more fully leverage benefits of the scaling trade-offs we made.

Being able to segment and slice the outbound data from your clusters (and/or what the client requests) - that is, the data and game state equivalent of occlusion culling - goes a long way.