I don't think I would entirely agree with that. Surely it depends on the developer. I can build a traditional CLI program faster than I can arrange a GUI for an equivalent program in something like interface builder although I've met developers who are faster the other way around.
Yeah a basic CLI is super easy. I mean at the sophistication required to be as usable as the parent post implies may be more difficult.
Do people build those GUIs without toolkits or GUI builders though? An advanced shell concept would presumably come with tooling too...