What's the speed of sound in a vacuum?
Sound doesn't propagate in a vacuum. By 'supersonic' in this context, I assume that it means faster than the speed of pressure waves propagating through the gaseous medium surrounding the black hole. Just as objects in our atmosphere can move faster than the sound they generate, so too can gas molecules exceed the speed of the pressure waves generated as they're pushed by the black hole's gravitational and magnetic slingshot effects.
(I'm not a physicist, but I play one on FR.)
The idea of "speed of sound" as presented in this article is silly. By its definition the velocity of the air coming coming out of the air vent above me could be considered supersonic once you add its velocity vector relative to me to its velocity vector relative to the sun.
Speed of sound has everything to do with the velocity of a fluid relative to a secondary mass it's capable of communicating pressure info with none of which is implied by the article.