Pretty much. DE works on some insects because it is a desiccant - it literally sucks all their moisture out and kills them. So if it gets wet, it can't do that. And it's very wet inside an animal, so the DE is saturated, and then it's useless for anything. Some people believe it's sharp and cuts...
I used Safeguard recently to worm my emus. I found that the dosage recommended was 15 mg/kg. I actually used the pellet style, and put it in a dish and had no trouble getting them to eat it.