> To be clear: there is NOTHING at all about sound in this article. The spectrum does not show a continuum from sound to light.

Except that a portion of the spectrum graphic is labeled “perceptible sound.” That’s what the OP is referencing.

Perhaps the illustrator is pointing out that the frequency range of these EM waves coincides with frequency range of audible sounds? Or perhaps they’re confused. Either way, there’s no mistaking that the graphic is labeled with the words “perceptible sound.”