Yep, this is definitely true! Many worms are spread through fecal oral route and some even need time until they become infectious (sometimes a few days) so the more often fecal matter is removed, the less exposure the animal will have to those parasites.
I agree with Stacykins. Generally, livestock (sheep, goats, cows with an exception being llamas and alpacas as ivermectin is very good for P. tenuis and DOES NOT cause resistance in that species) should be treated when necessary, meaning regular fecals or when they are sick. The problem with...
I agree with the others. DE, though popular, is really not a great choice for parasite control. A good control program (regular fecals, pasture rotation, worming only when needed, and hygiene/cleanliness) will work far better at controlling parasites.