Once you've figured out that mouse movements correspond to moving the pointer, clicking your left button usually means you want to do the primary interaction with whatever you're clicking on (opening a program, pressing a button, checking a checkbox), and right click gives you additional things you can do with the thing you're right clicking on, you've figured out enough to be able to at least use the system (I agree you're by no means proficient in it though).
To get comparable proficiency with a command line requires a much larger set of things to remember for even basic usage.
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.
it's easy because MS has spent years in formal user studies for almost every element of the 95 UI to make sure that it was easy for pretty much everyone.
Cosmetic differences in CLI applications are actually important to get them to work in the first place. dash vs double dash, equals sign or no equals sign. CLI parsing is completely nonstandardized. Each command reinvents parsing and is completely unpredictable.
The only way you could possibly solve this is by letting the shell parse CLI arguments and just pass them as a JSON like object to the application. The ship for that has sailed a long time ago.
You know the secrets on how to use the Microsoft Windows 95 GUI so it's easy for you. Consider the person who has not been trained over years in its use. The concept of a context menu that pops up at the mouse position when clicking button number two is not intuitive or discoverable.
My wife, for example, grew up before Windows 95 and its descendents were invented, holds multiple post-secondary degrees, and works with computers every day. She has no idea a second mouse button exists let alone that she can use it to perform file manipulation operations. She doesn't quite get the idea of "home directories" or "nested folders" or anything that is hidden like menus (or, frankly, tabs in the browser.. or multiple browser windows in fullscreen mode). Those things are not intuitive or discoverable in any way if you haven't been trained in their use. I, too, often forget the context menu is there although with practive I'm getting better. I certainly would never forget the `df` command though because it's second nature to me after over 40 years of use.
People need to be very careful when making an argument that something is better because it's what they're familiar with as if they are some definitive exemplar.