What is their criteria for selection? Are they working from the idea of a published piece of music that could be performed by any symphony, quartet, choral group, etc? Or is about what instruments it is for? I'm just not clear on what their definition of "classical" music is.
Classical music as a genre is quite hard to define. Especially if you try to find some clear boundaries of where classical ends and jazz begins. That is the case for basically every genre. Personally I would try to approach it like this: If it needs the particular band/artist to play it live, it's not classical, if it can be played by anyone with the notes and still be considered the same as hearing the original, it'…
There are other practical concerns such as being able to hire from a pool of musicians who can all do similar work. So it leads to a large repertoire that is hard to define in a formal sense, but develops similarities out of practical necessity.
There are other examples as well. The string quartet and the "big band" of jazz, where a particular instrumentation and performance setting guarantees a limitless supply of repertoire for the musicians, and a hope of getting performed for the composer or arranger. I play in a "big band."