For non-pros like myself, it's unfortunately not common to shoot with a camera that has good auto-ISO: basically anything above ISO 800 is going to be unusable, and ideally you want ISO 100 or 200 if your lighting situation can allow for it.
So most of the time I find myself in semi-manual modes: where I lock the ISO to a setting that's ideal for the light I have, use auto-focus (unless it's failing to lock on the correct target), and set either the aperture or shutter speed to what I want. The camera then adjusts the other factor appropriately, but I still find myself using Canon's quick adjustment exposure compensation tool almost constantly because my Canon blows out highlights pretty badly a lot of the time.
For a pro I imagine the situation is usually reversed: set both shutter speed and aperture how you want them, and trust your camera to be good enough with the auto-ISO. It sounds to me like GP was basically right: in a non-controlled setting, full manual mode is pretty rare.
The important distinction is that "manual mode" is being discussed and compared to "aperture mode." So it's about the PASM setting and not specifically about "true manual mode" (auto-ISO or not). The clear (and in my view incorrect) assertion of GP is that manual priority/mode is not for use outside the studio. It is said three times:
> I know almost no professionals who shoot on manual mode outside of studio settings.
> The only time manual mode is used is when you're in a studio...
> Leave manual mode for the studio.