It is obviously a technical term in music, though not familiar to me. It MIGHT POSSIBLY be an invented word, because her point is that these are "countervoices" and but NOT "counterpoint."
"Voices" in music refers to the separate melodic lines you can hear in a complicated musical texture. It does NOT mean human voices or singing.
Imagine that you are trying to hum the "tune" of a symphony. You would hum the part that's easiest to hear, usually the "upper voice." But it is not satisfying to hum a symphony, but you also hear subconsciously, and consciously if you pay attention, to other melodies within the music that are being played at the same time.
"Counterpoint" is music in which several melodies are played at the same time. Each can be heard by itself, but they fit tightly with each other like puzzle pieces. They create harmony at the same time as they create melody.. The harmony is very important, and you "feel" chords and tonal centers at the same time as you hear melodies. Counterpoint is common in Western classical music such as symphonies. It is very characteristic of baroque music, such as Bach fugues. It is also found in Sousa marches, Strauss waltzes, Broadway show music, and movie music.
The writer is describing music written for children by Carl Orff. She says this musical style is unusual. She is using the technical words of music theory to describe its characteristics. She is saying that Orff's music for children uses several different instruments playing different melodic lines. They sometimes overlap,, but she is stressing that it is NOT "contrapuntal." They do NOT fit together like puzzle pieces and create traditional harmony. The harmony is not very important. The music is "about" the melodies, not the harmony.
An "ostinato" is a repeated musical phrase, that becomes easy to hear in a complicated musical texture because it is repeated over and over.
To be continued...