Since I posted my comment I checked Wikipedia, so I'm now professional in this area . The wiki article mentioned, that USB HID actually supports NKRO, but not during the boot (boot protocol vs. report protocol). So it seems, that the Reddit comment in the video was partially wrong, because as you wrote, the keyboard controller has to support this feature and it can work with both USB and PS/2.
It's interesting to see your comment, I think this is the first time ever somebody had encountered an issue with the Model M keyboard. Unfortunately, I couldn't experience it myself, so I have no own opinion. My first keyboard was a standard keyboard for a Siemens-Nixdorf PC with Pentium processor, which also had PS/2 connection and as I remember it couldn't handle too much multiple keystrokes.
So it seems as for everything in IT, we can say: "It depends"