While it is true that protein is a biological necessity for dogs, they are able to derive limited nutrition from certain types of carbohydrate. (pawster.com)
As you have already learned, dogs have a limited ability to digest plant products but they can derive some nutrition from certain types of carbohydrate. (pawster.com)
You might want to speak to a doctor / naturopath / dietician about it, the diet focuses on avoiding certain types of carbohydrates. (deliciouslyella.com)