Some body (who was bored I assume) once posted online that animal digest is crap..literally, and/or that it was made from 4-D animals. Which is what you mentioned. This definition even made to wikipedia, although wikipedia is often known for not always having the facts. All animal digest really is is animal tissue. I am not sure how that rumor spread as quickly as it did, but if your animal is eating animal digest it would not be the end of the world. The digest likely would come from whatever animal is the main ingredient as most factories would be unwilling to pay extra for just the tissue of another animal when they have plenty from the original.

Hope that clears things up for you!