Corn is not good for some dogs, fine for mine. OP, if your dogs are currently doing fine on a food with corn then there is no need to exclude it, other than, as mentionned, it's not great as the first ingredient. In my readings chicken is the leading allergen in food, followed by wheat.
I would like to point out, be aware that terms such as "holistic" on dog food have no legal meaning. When you purchase on the basis of that word you are going solely on your faith in the company using it. It might be meaningful, it might not. It's a marketing ploy and as such is easily used by any company since it can mean whatever they say it does. And it may not mean what they say it does.
You are right, it is mind boggling.