But the author makes a larger point, that even the stuff people have grown to like was often considered outre and difficult in its own time, and that people have gradually grown accustomed to some amount of dissonance.
"Classical music is, by contrast, a tightly controlled club with a bunch of smart guys deciding behind a curtain what is and is not legitimately classical."
Really? I've never gotten that sense. At least form what I've read or heard from composers such as Michal Nyman, Philip Glass, Nico Muhly, John Adams, they all see pretty open that what might be called "classical music" and really only seemed concerned that their own stuff might get pigeon-holed (e.g., tagged as "post-minimalism" or something). Shows on NPR, for example, are also quite broad in what gets included on the club.
Maybe some critics or authors have some scheme to control the meaning, but they seem to be doing a poor job of it.