Pipe Organ “Answers” in Plainsong?

In 2003, I copied a book by Félix Bélédin (d. 1895), who was titular organist—from 1841 to 1874—at the Cathedral of Saint John the Baptist in Lyon (France). In 2008, we scanned and uploaded the book to the Lalande Online Library. Nobody knows for sure when the book was published; some believe it first appeared in the 1840s. In any event, one who examines this excerpt, showing GLORIA IX might wonder why it says the organ answers in plainsong. However, the front of the book explains, telling the organist explicitly when to “respond in plainchant.” This is something called organ alternatim. Believe it or not, the pipe organ would take turns with the choir, playing certain texts instrumentally instead of having them sung. I’m not very well-versed in this—pardon the pun—but if memory serves, ORGAN ALTERNATIM was frowned upon by the time of Pope Saint Pius X. Nevertheless, French organists kept doing it, even after it was explicitly condemned as an abuse.