How does hydration affect digestion?
That's not a typo. The author is talking about cooking polysaccharides (gelatinisation), which in fact increases the digestibility:


  Gelatinization refers to the irreversible loss of the crystalline regions in starch granules that occur upon heating in the presence of water. The temperature range during which the crystalline structure of the starch granule is lost is dependent on the water content, and on the type of starch. The gelatinization dramatically increases the availability of starch for digestion by amylolytic enzymes. (emphases mine)


Source: FAO: Effects of Food Processing on Dietary Carbohydrates.

You can read more about it in this Wikipedia page.