That thread doesn’t explain anything, and again, nobody there differentiates between USB and PS/2 keyboards.
Looking it up, however, it looks like not all keyboards took advantage of PS/2’s superiority.
Viewing post in A plea to developers. Don't only use WASD.
I think you are missing the point. It doesn't matter how many keystrokes the interface can send if the keyboard itself can only detect 2 keys being pressed at a time. The thread explains that many keyboards at that time could only detect 2 simultaneous key presses not counting modifiers. At the bottom it covers the technicalities of why that was, i.e. keyboard manufacturers going from having each key individually wired to using a grid system to cut costs and having to impliment anti-ghosting due to the grid system's limitations.