From the source quote:
As I noted, we gave up the big guts necessary to retain it long enough in a lengthy digestive tract in favour of using that energy to support a big brain instead. Our gut bacteria are not digesting cellulose. I think you are referring to ‘soluble fiber’. Gut bacteria are digesting other polysaccharides that we metabolize.
Soluble fiber includes gums, pectins, mucilages, and some hemicelluloses. According to the FDA, soluble fiber is listed on food labels as having calories because it does, in a roundabout way, contribute calories to the body. This is because most soluble fiber is used by the bacteria in the colon to produce short-chain fatty acids which, in turn, are used by the body as energy. These calories do not raise blood sugar, so when counting carbs, those in soluble fiber (like insoluble fiber) don’t count towards the total. This is also true of oligosaccharides, which may or may not also be listed as fiber.
Insoluble fiber includes cellulose, some hemicellulose, and lignins. It can be found in the seeds and skins of fruits and vegetables as well as whole grains like whole wheat and brown rice. It cannot be used by the body for energy.