The delineation between reverberated drone and sustained tones is interesting, and there is definitely a big difference in the aesthetic, and there's also people like Windy & Carl, Kyle Bobby Dunn and Celer who use guitar and loops to create dronish music. Ive heard the sound described as 'ambient drone', which I think is a subtle but important distinction.
I guess Id see the major qualities that distinguish drone from ambient in general as being around approach, intent and genealogy. Whereas ambient is a fairly amorphous fusion of ideas from new age, environmental music and the avant garde, 'hardcore' or pure drone is a much more restrictive sub genre of classical minimalism that is based in ideas from the academy. The originators of the modern genre - the likes of Phil Niblock, Alvin Lucier, Radigue, La Monte young etc. all come from a classical avant garde tradition, which is reflected in the austerity and intensity of the music. Drone isnt music designed to be 'ignorable'. Its intended to be a serious, radical offshoot of experimental modernism.