Musician Catherine Schmidt-Jones defines art music as "a music which requires significantly more work by the listener to fully appreciate than is typical of popular music". In her view, "[t]his can include the more challenging types of jazz and rock music, as well as Classical".
I took a look at the Wikipedia article on Art Music.So, here's the thing... vague terms such as these are not amenable to precise definition. They will be used in many different ways, to mean many different things. And none of those meanings may be considered 'wrong'. Unlike in the hard sciences, definitions need to be taken with a grain of salt in most other disciplines.
Initially, Wikipedia states the definition of art music which you describe. But then, under the header "Definition", are listed a number of similar, but slightly different characterizations of what "art music" constitutes. In particular, I'll quote the one you mention.Now, this definition is clearly different from the one at the beginning of the article. And according to her understanding of what art music is, the more challenging types of jazz and rock qualify as well.
I think that at the core of the distinction is a difference in intent. As I think of it, art music is music which is meant to be listened to "actively", as an entity in its own right. This is generally not the case with popular music.
I wouldn't say that art music is entirely a relative phenomenon, either. Imagine that you are suddenly in an unknown country. How hard would it be to distinguish art music, folk music, and popular music? I would suppose it would not be very difficult.
As with all things, there will be music which is hard to classify. I don't think even musicologists say that any song can be easily classified as art music or otherwise.
Just skimming through the thread, it sounds too generalized to just split music into art, pop, and folk. I think some opera (Puccini or Verdi I think) composers were "top ten" music composers.