It's not covered.

>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.