It is covered for both X11 and Wayland. I just don't get into the particular decisions and details of how each WM/DE picks what they deem the current focused window, since it varies widely and it's more part of window management than of input management (I've written a WM and it's a bit messy). An article on WM/Compositor development would be more appropriate for that, I have a few already on my blog.
>Obviously, each have their own internal representation and ways of managing the information they get from libinput, XKB, and others, but this is outside the scope of this article
From what I can tell this is the explaination. It says that they manage input. If this is good enough why not simplify what the kernel does by saying Linux has its own way of handling inputs from keyboard and that is out of scope. It just seems arbitrary what handling of input is and isn't in scope.