Not every useful fibre ferments strongly
Psyllium is often discussed as a gut fibre, but its value is not only about feeding microbes quickly. Compared with some highly fermentable fibres, psyllium is usually considered less aggressively fermented. For many formats, its more visible behaviour is water holding, swelling, viscosity, and gel formation.
That still matters for the gut environment. A fibre can influence stool moisture, texture, transit, and the physical contents of the colon without acting like a fast microbial fuel. This makes psyllium different from fibres such as inulin or some resistant starches. The right question is not whether psyllium is prebiotic in a simple way. It is what behaviour the application needs.