What they are talking about is that some of the Wayland compositors rely on things like libinput and libdrm which are Linux specific.
This is not “Wayland” really but, from the point of view of a regular user, it may as well be. As the OP points out, there is no /usr/bin/Wayland
It is not really a great criticism although it must be frustrating for the BSD folks and others. Of course, the answer like always is to contribute. Nothing stopping anybody from taking wlroots ( or whatever ) and adding abstractions that make it more portable.
Non-Linux operating systems have already added Wayland support ( like Haiku ). If I had the time, I would add it to SerenityOS myself.
Actually, if I had the time, I might write a WaylandServer for X. First, it would be funny. Second, the people that do not want to move could stay on X forever even when everything stops supporting it. I would have to make sure that my WaylandServer could run XWayland of course.