They cannot digest it.
There have been many studies using acid with the same acidity as stomach acid, and it takes way too long for the calci-sand to digest. While the one batch is digesting, the reptile continues to eat it, and the impaction has been created.
The sand does contain calcium, which makes it more enticing to eat, increasing the risk.
The product is not recommended by any reptile keeper who knows anything about the care of reptiles.