Primordial content in Beethoven

The Three-Toed Sloth blog writes about a confusing, and perhaps confused concept:

Here is Martindale's Figure 9.1 (p. 288), supposedly showing the amount of "primordial content" in Beethoven's musical compositions from 1795 through 1826, or rather a two-year moving average of this:

Let us leave to one side the very difficult questions of how to measure "primordial content"; Martindale, like too many psychologists, is slave to quite confused ideas…