In the end it is not very different: a display server must present an interface so that clients can draw things on the screen.
The only thing is that all Wayland compositors use the same interface, defined by the Wayland protocol, while Mir uses its own interface, defined by its API.
Although wayland is only the protocol, it has strong implications for the architecture and the design of any implementation of it.
That is why you can compare them directly (instead of a specific implementation) regarding technicalities and architecture.
Also, it will ensure compatibility between clients and compositors following the protocol, so Mir and Wayland can be considered as two blocks when comparing adoption and the like.