You're comparing a full-time salaried employee with a "few months" to spare vs. a bunch of people spending their spare time. Obviously not the same at all. There is a difference between "able to do the task, given enough time" vs. "wanting to invest the time". In a volunteer context "I don't have the (time|skill)" usually means "I don't want to invest the time".

In addition, making a WM in libX11 is surprisingly easy, almost trivial. With Wayland: not so much. Maybe the protocol is simpler for some meaning of "simpler", but actually doing stuff with it is often more complex, certainly if you want to write a WM (or compositor in Wayland-speak). I managed to write my own X11 WM without any real experience doing this sort of thing that has served me quite well for the last ten years. I also tried duplicating that in Wayland because I've been told that's the future, but I found that a lot harder to the point where I kind of just gave up. While I'm sure I could figure it out given enough time, I also don't really want to as I have other stuff to do, and also have a perfectly working X11 WM already.