I think he is wrong about Soylent but not because Soylent explicitly optimized for this eventuality. Soylent happens to use oat flour which is rich in resistant starch. This is exactly the type of "difficult or impossible to digest" thing that the bacteria in our gut feed on.

Processed food's association with colorectal cancer is not related to the bioavailability of its nutrients or to the presence or lack of insoluble fibers in the diet AFAIK.