By window manager started out as ~50 lines of Ruby copying an equivalent amount of C.

You can say many things about Wayland, but it's "simple" from a point of view I for one really do not care about. Wayland may be "simple" in some respects, but it makes most of the things I care about doing unnecessarily complex.

Lacking features isn't the same thing as "simpler", Wayland is great, but is very much a subset of the features implemented on an X11 desktop. Wayland doesn't do selections or provide any IPC mechanism of its own, much less something like an ICCCM that allows you to identify/target other users of the desktop and interact with them in a flexible way. In fact as I understand it the linked tool is in fact impossible to write in Wayland.

Again, this isn't the fault of "Wayland", which is just a compositor framework. The complaint is that the ecosystem of "desktop" software which evolved around Wayland is an ad hoc monstrosity that lacks the unified structure that its ancestor had way back in the X11R5 days.

Wayland is simple because it shoves all the responsibilities to somewhere else.