Nicola,
Not sure that what is quoted demonstrates that something is permanently sticking to the sides of the colon. There will always be a small amount of material that is not expelled, just as there is a small amount of toothpaste left in the tube that just can't be squeezed out but could be washed away if water were run through the tube. This doesn't mean that there is a thick layer of build up of material that coats the colon and can only be removed by magic herbal remedies.
Think about getting mud, dirt, or grease on your hands. As much as you'd like, you can't remove all of it by squeezing your hands together. Due to the irregular surfaces, there will always be a small amount that will stick. If you add more, it doesn't build up, you're still able to remove all but that last amount. Same with the toothpaste tube. You can add more toothpaste to the tube and again squeeze all but the last bit out. It doesn't build up.
The more liquid something is, the thinner the remaining layer. So in the case of dirty hands, the toothpaste tube, or your colon, if you thin the material with water, some will dissolve and flow away, but no matter what you do, some residual will always remain. Even pouring water over a vertical surface will leave that surface wet with a layer of water. Pouring more water over the vertical surface does not create a build up. The amount remaining is determined by physical properties like surface tension, adhesion, capillary action, viscosity, etc of the material as well as the properties of the surface that the material is on.
Lex