I started to type out a lengthy nitpicking response about this, because it's a fun conversation to have, but I think I was disagreeing for the sake of. Mostly with phrasing. Is the evolution of blues into something white people renamed (subverting a black term) and changed and dug an "innovation"? Not in the sense that slide guitar was, or whatever. But music as class equalizer didn't exist until rock. On the contrary, music had in the past been sometimes the opposite. Good answer. Oh, and, I didn't read the article but what's the deal with the radio? Radios predate rock by a good while. I'd point to the post-war invention of the teenager as being a hell of a lot more important as a societal factor in the popularization of the genre.
I think, in the context of the article, it's less of an "innovation", and more of a "What made rock and roll an important force in the 20th century?" I think they just chose a bad descriptor. if you take that context, too, then the answer "the radio" makes more sense, i think.