They've been satisfied enough with the GUI because the alternative, the command line, was several orders of magnitude harder to learn than the GUI. Siri is comparable, or possibly easier, in terms of learning difficulty.
Fact is, with a command line you don't "talk" to a computer. You actually program it
But with Siri, you talk to it. That's where the NLP piece comes in. "Wake me up at 7am" works, so does "Can you set an alarm for 7am?", etc. The syntax is deliberately loose and not programming-language-like. So you don't need to be exact at all.
A CLI is an interface that lets the user communicate using the the computer's language.
Siri is an interaface that [attempts] to let the user communicate using their own language.
It's a fundamental difference I think. Indeed the only thing that Siri and CLIs have is that they are non-graphical.
This is actually the similarity that matters. In a GUI interface, you have to page through "commands" to find what you are looking for. In CLIs and Siri, you simply go straight to the command by typing or speaking.
Programmers love CLIs because, after the initial cost of learning the commands, you can do things much faster than a GUI. But, speed of executing commands is less important on the desktop than on a phone, which is why I suspect most normal users don't bother to learn a CLIs.
But when you're on a phone with trying to tell your boss you're running late while coming down a flight of stairs, Siri is going to come in real handy.
The main reason why most users don't use a command line isn't, as the article says, because "most people have been satisfied enough with that [the GUI], so they haven't ever bothered learning to use a command line". It's because command lines are only better for very advanced users and/or people with an exceptional memory.
Fact is, with a command line you don't "talk" to a computer. You actually program it, with a kind of interpreted language and a plethora of libraries (executables). This means that you actually need to be VERY exact in what you write, you have to remember the "magic incantation" in every detail, or it will not work - or, likely enough, disaster will follow.
Vocal commands can be useful for the mass for very simple tasks, if the incantation is very easy to remember. To be able to actually talk to computers, on the other hand, we need actual AI, of the Turing test kind. We aren't anywhere near that for now.