That Nvidia FAQ is NOT stating that stutter is caused when framerate drops beneath refresh rate. It says it right there in that quote you used,
"When using vsync" are the pertinent words. They are in this example referring to traditional vsync methods where when your framerate drops beneath your refresh rate, vsync will then lock your framerate to the next step under that, often 30fps. That vsync jump from 60 to 30 is the stutter that is referred to.
They go on to state adaptive vsync is the fix to stuttering but not input lag,
Adaptive Vsync is a newer method of commonly used vsync where framerate locking is enabled when frames are high, above the refresh rate, and it is disabled when framerate falls beneath the refresh rate, allowing the framerate to naturally settle or shift under the refresh rate. This right here is a statement that falling beneath the refresh rate when using adaptive vsync, or indeed when not using any vsync, is not this cause of stuttering they refer to. In the very beginning of the quote you used they state with shifting framerate the concern is with screen tearing, to which that early form of vsync was introduced as a fix, with the stuttering a result of that form of vsync.
This is ignoring my opinion that the words stutter and hitching are often used interchangeably and their definition in gaming is not nailed down that firmly.