I don't think this proves anything. For example there is a
massive difference between people using shortcuts in an everyday application, people used to Vim/Emacs/whatever to a point they can do the basics without thinking about them, and people who actually customized their setup for their workflow.
And to me it looks like they only focused on the first case because of sentences like this:
> It takes two seconds to decide upon which special-function key to press
Yes, I can definitely believe that I wouldn't be faster than a mouse user if I had to select text with a keyboard in notepad or press Ctrl-Alt-whatever to do simple tasks. But good luck beating a proficient Vim user at anything that has to do with cursor movements, search/replace or repetitive steps - watch a competitive coder use Vim and see how often it takes them "two seconds to decide" to do anything.
I think overall people have a decent intuition about what the more efficient way to control an application is. I frequently use the mouse/touchpad too, because in many GUIs it's simply faster and you can feel that even when you start out focused on the keyboard. Often users switch between window management and using graphical applications, and in those cases leaving the hand on the mouse is the more convenient approach.
And lastly - does it even matter? When someone perceives a certain approach to be faster or smoother, then why not let them? In almost all cases the speed difference doesn't matter.