Answer:
This claim is marketing to try to set themselves apart from others. There is no evidence that the millions of doses of supplements consumed by humans and animals every day cause harm. There is, on the other hand, tens of thousands of scientific and clinical papers and research proving the exact opposite. If such fortification was harmful or unnecessary, why would the entire pet food industry, other than this one marketing company, do it?

Please note this quote from one of Dr. Wysong's books, "You can find articles about how this or that vitamin is toxic or worthless. However, all such reports, upon analysis, are based upon poor science and manipulation of data. This bias in the popular media against supplements is fostered by the medical and pharmacological Industries that wish the public to be dependent upon them. They see self care and supplementation as a threat to their economic success. The media, supported by huge advertising dollars from Big Pharma, omits the very real fact that modern health care is the number one killer. At the same time, there is not one indisputable death caused by the consumption of supplements taken at appropriate levels in either humans or animals."

There are over sixty billion doses of vitamins taken by people every year, yet not one fatality. There are about 160 million pets (dogs, cats, horses, birds) in the US eating vitamin and mineral fortified foods about twice per day, not including the supplements given to animals in addition to their fortified foods, the doses to animals would be over 112 trillion every year. Yet not one fatality as a result!

The ability of any company to market products based upon the opposite to these facts speaks to the power of mythology.