Once learned the GUI actions are impossible to forget. Being away from the computer for two weeks will make you forget 80% of the CLI commands from the author's list. The reason is simple: the brain can associate the GUI actions to many concepts from the real world. CLI concepts exist on computers and nowhere else. That knowledge is too abstract and too expensive to be kept unused in our brain's cache non-stop.
As someone who uses both GUI and CLI, and has taken month-long travels where I don't use a computer at all, I don't think this is true, at least for me. CLI is like a language; once you've really learned, it takes a long time to forget.