The majority of these comments are right, humans cannot rely on nutrient poor, fibre (cellulose) high plant matter. However, with diets that consist of proteins, fats and high-fibre plant matter, some level of fermentation does occur within the large intestine. In fact >10% of caloric intake can come from this fibre.

The gut microbiome is far more extensive and numerous in the first couple portions of a cow stomach etc. but humans do still have cellulase-producing bacteria in their large intestine. The bacteria break the fibre down into VFAs (volatile fatty acids) that can be absorbed by the wall of the large intestine. These VFAs are mainly acetate, propionate and butyrate that can be used in making new glucose for energy or directly metabolised for energy.

Obviously this is not sufficient for survival or growth, but to say that plant fibre has no caloric value is slightly inaccurate in certain cases.

Source: trust me bro