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.
The speed in units is irrelevant; what's at issue is where the shockwave phenomenon occurs. The equivalence between "supersonic" and 340.3m/sec. is only valid at room temperature and sea level, but it's close enough that airplane and spacecraft speeds are expressed in Mach units.
I agree that more precise terms exist, but "supersonic" is not flat-out wrong -- just vague to a degree that I would not consider acceptable in a scientific journal, but which is good enough for an article directed at a general audience.
You’re splitting hairs, needlessly. You know they’re using “the speed of sound” to represent the accepted constant of 1088 feet per second used in mach units.