need to read into it a little...
As the chimps were probably chewing on whole shoots. You would obviously observe them chewing on the pith, buts its the bark/saps that get swallowed for the most part.
Of old cultures that keep their teeth into old age you will see them chewing on Licorice root: Not only does the pith brush the teeth, but the Glycisides in the bark act as breath freshener, antibacterial, and heal the gums.
The inner bark of trees, shrubs, and roots are where all the Bioflavonoids, and glyconutrients (Polysaccharides), and soluble fibers are - as this is where the sap flows to provide nutrients for the plant. In olde England other Elms or specific Willows were used for this (note other willows were used for an early source of Salicylic Acid to use for headache relief).
Slippery Elm Bark and Red Cedar were both used in days of yore for food, Baby formula.
Tannins, herbal medicines (like Berberizine from mahonias, or Goldenseal root) come from root/branch barks if not the using the the leaves/herb or flowers.