That's like saying every level of a game is an instance. That's retarded. An instance is the exact same art assets (NPCs and all) being re-used among different groups of players. In other words, 100 people in 100 different versions/copies of the same cave. Sharding and instancing are practically the same thing, but loading between continents in WoW is neither because you don't have two versions of Kalimdor, only one, and it just takes a minute to load.
This ^
But to be fair, a phase and an instance are almost interchangeable also. Phases are just larger versions of instances really.