I very much agree. The author appears to have defined 'avant-garde' chronologically, rather than with reference to the music itself. I just don't think such an approach is really tenable after about 1910.
And, in an ideal world, shouldn't be tenable at all!
I still like the term, for all that it is problematic and has been subject to varying definitions. There's something to be said for Peter Bürger's characterisation of specifically 'anti-art' movements (especially Dada) as constituting the true 'avant-garde' - Greil Marcus has written about a type of canon that leads from this through the Lettrists and Situationists up to the punk movement - maybe it's trying a bit hard, but this canon isn't unmeaningful. In a post-war German context, I use the term more broadly, simply to refer to a species of music that constitutes a palpable break with that of the preceding era - so that the sort of flirtation with twelve-tone music on the part of those who continued to inhabit a neo-classicist/Neue Sachlichkeit aesthetic (including the post-war Fortner, Henze, much of Klebe, and various others) does not really amount to an avant-gardist strain, whereas the work of Eimert, Stockhausen, Koenig, Schnebel post-
Versuche, Riedl and others is. B.A. Zimmermann is a difficult composer in terms of these categories - it's not really until the 1960s that his work inhabits a comparable space to the previous avant-garde, by which time many of them had moved onto other things; even a monumental work like the
Requiem doesn't seem to constitute such a break as Stockhausen from
Kreuzspiel onwards (or Boulez after the first two piano sonatas and flute sonatina, or Schaeffer and Henry, or Xenakis, or Cage after 1951) did. Nothing is wholly divorced from what came before, and hardly anything doesn't do something new, but the difference in degree makes the categories workable, I believe. Certainly they were meaningful in terms of the aesthetic positions of some of the opposing camps in the 1950s.
I find the way you've engaged with the label far preferable to the way I interpret the author's, in that you ground your definitions in the aesthetic and musical content of the works themselves, rather than simply in chronology.
I just tend to find that applying labels in any kind of prescriptive sense reeks of that model of teaching that says "Xenakis was born in 1921 (or 1922) and therefore he writes avant-garde music". Or, at its worst "Beethoven was a composer in the Classical period, until he invented Romanticism". Labels certainly have their uses, but those uses grow firmly out of an analysis and understanding of the musical text itself, and the individual composers involved.
Ultimately, all of the composers you name interact with what might be described as 'avant-garde concerns' in radically different ways. That doesn't mean that they can't be described as being avant-garde, but it does mean that to describe them as such and leave it at that does them a disservice. In a similar, but more specific vein, the 'New Complexity' appellation (which I'm sure was delivered with the best of intentions, and by people familiar with the real issues involved) resulted in the grouping together of composers as a 'School' based almost purely on the perceived difficulty of their notation, which is surely almost offensively superficial.
