X11 and Wayland are both protocols.

Xorg is a display server. In Wayland, your compositor is the display server.

“I have run graphical multi-seat systems using x11. Something like that will never be possible in the same way for Wayland”

I have to give you this one. Wayland is not designed to be multi-seat. I do not know about “never” but you are right that multi-seat is a design difference.

My mind goes to this project again: https://github.com/wayland-transpositor/wprs

But wprs only runs one compositor so it does not inherently address multi-seat. Support for that would need to be added.