Jojo, while they do ingest foreign matter from time to time incidently, it would occure far less often in the wild, as Syn points out, than it will in the captive environment, where it cannot move off.
The point to keep in mind about impaction is that it's not always an immediate thing, e.g. lizards eats crap, immeadiately gets blocked up. If you closely inpected every dropping, im sure you would find, as some claim (especially with females/laying media), that their lizards often pass matter such as soil, sand particles etc.
Thing is that you cannot be sure that
all ingested foreign matter is passed out.
Some, like sand particles for example, can and do remain in your lizards gut/intestinal tract for long periods, slowly building up to a fatal impaction. Necropsies have proven this
with lizards that seemed just fine for a very long time, only to crash suddenly.
Statistically, your captive is far more likely to have issues either in the short or long term, than any wild animal. Cup feeding does not garantee no ingestion.
Hence, common sense prevails.
An ounce of prevention is worth a pound of cure!
p.s. I could search the site for example threads, but then, so could you.
p.p.s. I have seen plenty of impactions from various substrate media among a mix of species over the years, quite a few fatal.
A lizards digestive enzyms are powerful, but this does depend on the state of health/metabolism etc, and some material is simply
indestructible. Physical size/shape of ingested material also plays a part, some being more prone to cause issues than others.
Hairballs dont break down in a cat or rabbits stomach, I imagine coconut fibre would entangle and compact over time in much the same way.