One of the reasons why Wayland is "simpler" than X is that it doesn't include the APIs that window managers like Xmonad need. Instead, window managers have to implement a lot of the complex low-level graphics details themselves. This is why Wayland window managers are called "compositors" -- they include a lot of the code that used to be provided by the X server.

So the game development analogy here is, like, someone has just made their first Unity game and it's pretty good, but they don't feel like they'd be able to write their own game engine that's as good as Unity.

(The article mentions wlroots, which was an attempt to write a more general-purpose helper library for Wayland compositors. In practice this didn't work out; every major compositor other than Sway writes their own implementation of the stuff wlroots does.)