That's not the same thing. It's way easier to write an X11 window manager than to write a Wayland compositor, even with something like wlroots, because the window manager can speak the same protocol that clients speak, and it runs as a separate process.
As a concrete example, Emacs' EXWM package works by implementing an X11 client library in Emacs Lisp, then using it to talk to the X server (which is a separate process, so this works fine) and telling it how to position windows.
Whereas on Wayland, this is not possible without re-implementing a standalone compositor process, because otherwise architecturally it doesn't work. Emacs can't both do the drawing and be drawn.
As a concrete example, Emacs' EXWM package works by implementing an X11 client library in Emacs Lisp, then using it to talk to the X server (which is a separate process, so this works fine) and telling it how to position windows.
Whereas on Wayland, this is not possible without re-implementing a standalone compositor process, because otherwise architecturally it doesn't work. Emacs can't both do the drawing and be drawn.