what if your mouse doesn't have two buttons? In fact, some mice don't have any buttons. (apple, over the years)
That's apple's problem. I've been fixing apple devices since the puck mouse. They choose to go against the grain on purpose. Sometimes it makes sense and sometimes it makes absolutely none.
...and, to build on this: the fact that this is Apple's fault means that it's not the fault of the GUI paradigm - just Apple's implementation of it.
It means it's not the fault of the Windows 95 GUI paradigm most middle-class North Americans millennial age or younger have been trained with. Windows 95 is not the GUI paradigm, it's one single product that implements a GUI paradigm.
That is not the argument they made, they said right-click acts as a "tell me what I can do with this file" discoverability tool, not that right-click is itself discoverable. There is approximately no way to discover this on a command line, but if you right-click and "play with Windows Media Player" is there, it tells you something useful.
But I'm going to make the argument that it is discoverable. Why isn't the context menu second nature to you after over 25 years of use? You put your hand on the mouse, it has two buttons, you click one of them in ordinary operation, what kind of hyperbole is it to say that "clicking the second button does things" is "not discoverable in any way"? You discover that by accidentally mashing it one day, if nothing else.