Sounds like snake oil to me.
Any product that says "Heals everything and more" is just that, ...... "Snake Oil". Stay away from it. If your dog's system needs cleansing, let your Vet prescribe it.
Yeah let them prescribe tons of drugs to treat ailments that are simply occurring because of the toxins in the dogs environment and being absorbed into the dogs body. Perfect case is environmental allergies. You could do benadryl and steroid shots every day for the rest of the dogs life, or you could detox the dog making it's body more able to deal with the allergies itself. No traditional vet knows how to detox a dog, holistic vets should.
The clay, i don't know anything about it. I've never noticed it in either of those foods and i carry both of them, but i certainly don't sell alot of them. So who knows what it does. However it can't be THAT bad since Nature's Variety is using it, and they're a very reputable company.
Nature's Variety's definition:
Montmorillonite is a clay that is primarily colloidal silicate, which contains over 50 ultra-trace mineral compounds including Antimony, Barium, Beryllium, Boron, Bromine, Cadmium, Carbon, Cerium, Cesium, Chloride, Chromium, Cobalt, Copper, Dysprosium, Fluoride, Gadolinium, Gallium, Germanium, Iodine, Lanthanum, Lithium, Manganese, Neodymium, Nickel, Phosphorus, Rhenium, Rubidium, Samarium, Scandium, Silicon, Silver, Strontium, Sulfur, Tellurium, Thallium, Thorium, Tin, Titanium, Vanadium, Ytterbium, Yttrium, Zinc, and Zirconium. Feed studies by the Veterinary Medical Diagnostics Laboratory at Texas A&M University have shown that Montmorillonite clays can sequester (bind) aflatoxins contained in grains and oilseeds.