The solar wind isn't a vacuum. The article is about the behavior of a stream of particles; the presence of those particles themselves makes it, by definition, not a vacuum.
The terms known as "vacuum" and "zero-gravity" are good enough for the vast majority of uses, but "near-vacuum" and "microgravity" are more strictly correct.
The behavior described is the same as that of air particles in the Earth's atmosphere. The particles at that speed can't get out of each other's way, forming the shock wave that's known as a sonic boom. Can the resulting vibrations be called "sound"? That's a semantic question. The solar wind is like a tree falling in the woods with no one to hear it.
Insofar as there is really no perfect vacuum in nature (even in deep space hydrogen particles exist even if far apart) I still say that the use of the term supersonic is ridiculous.
The solar wind is nothing like atmospheric wind. There is nothing even remotely approaching the density, and even the term supersonic would vary with density as the speed of sound does.