I guess what I'm trying to get at is, a game engine contains a 3D (or 2D) renderer, but that's not it's primary focus. There are other projects which are just that, a game engine is a lot of other things too, physics simulator, audio engine, UI framework, etc. A compositor is similar, it's a compositing window manager, a Wayland display server, an input handler, etc. It does far more than manage windows. When we look at X11 all a window manager does is manage windows, composition was a separate application, although sometimes the window manager would handle it, input handling was managed directly by the X server. Basically a Wayland compositor is a compositing window manager, and display server combined into one, it's like a game engine, calling one a 3D renderer is missing out on the vast array of other responsibilities.