Just playin Devil's advocate, here...
The issue isn't that it's not digestible, it's that a mouthful of it can, in rare cases, be harmful. For example, if it gets in the snake's mouth and festers, an infection could develop.
True, but in the wild they usually are not on loose substrate; rather, they are on plants, branches, and hard packed dirt. Additionally, the prey they consume would not usually be wet and therefore various debris would not stick as easily.but think in the wild, how many times do you thing the get dirt in their mouth?
That said, I like coconut fibre. It is a renewable resource, resistant to bacteria and mold, but I find it needs to be kept damp. If you let it dry out, it definitely does start sticking to things.