Impaction is NOT all that rare and can be caused by many different substrates. I've lost baby cornsnakes from swallowing sand and vermiculite and I've lost baby ball pythons from swallowing chunks of cypress mulch. Impaction is caused by swallowing a non-digestible item that is too large to pass though the digestive tract. The smaller the snake, the bigger the potential problem. The more substrate swallowed the bigger the potential problme. Coco husk is made of very small particles that will likely pass just fine provided that it's not a large amount clumped together. Also if your snake is an adult it's not likely that much will hurt it by being swallowed since it's digestive tract is much larger and capable of passing larger pieces.

Wood products like wood chips or paper are all made of cellulose which is NOT digestable. I don't know about coco husk, but I believe it breaks down fairly easily.