This sounds to me like they are adding in herbs/spices/essential oils. Especially since they also reported changes in the birds' demeanor as well. There are herbs/spices/essential oils that help to repel and even kill insects.As for feed additives to combat mites, I've just read about one they've tested in France called "PARALICE," but I can't find very good info about what it actually is, beyond "an in-feed patented non medicated mixture of aromatic extracts."
http://en.engormix.com/MA-poultry-i...novative-approach-tackle-red-t1528/165-p0.htm
The recommendation with the PARALICE is to add it to feed for a few weeks, a few times per year. No egg withdrawal necessary.
I can see several theories with this product. They could have found a combo of things that make the chicken's blood taste gross to the mites, keeping the mites from adequately feeding and reproducing. This is the theory behind feeding garlic to dogs to repel fleas, but it doesn't really work that great. Or they could have simply enhanced the birds' own immune systems with the addition of these herbs, that the birds were able to deal with the mites better.
Personally I use herbs/spices as well as DE in my own chicken houses and it does help with bug control. I also use essential oil mixes, commercial and homemade, with the animals as well as in the garden.