Part of how GLP-1 receptor agonists reduce appetite is by slowing gastric emptying — food stays in the stomach longer, so fullness lasts longer. It is a pharmacological effect, and a powerful one.
Viscous soluble fibre does something mechanically related, at a fraction of the intensity. Konjac glucomannan, among the most viscous soluble fibres identified, absorbs many times its weight in water and forms a gel — physically occupying volume and slowing how quickly a meal leaves the stomach. It is not a drug, it is not comparable in strength, and no serious person should claim otherwise. It is also food, it costs a couple of dollars a day, and it requires no prescription.
This is the premise behind nuverdin's 16-in-1 Daily Botanical Powder: four fibre sources built around exactly that mechanism, combined with botanicals traditionally taken around carbohydrate-heavy meals, in a single pre-portioned stick designed to be simple enough that people actually keep taking it.