As your quote points out, cows and sheep aren't meant to have that much corn. Humans can. Many cultures (traditional Mexican is the obvious one) incorporate large amounts of corn into their diets. One source says traditional Mexican adults generally eat a pound of masa (lime treated corn) a day. I believe some of the cultures that eat a lot of corn do not eat masa or corn that has otherwise been treated to release more nutrients, yet they do well.

But heirloom stock, organically grown, low-processed corn is a far cry from hybrid (bred for yield and storage not nutrition), chemically-laden, over-processed corn. Even masa harina (masa flour, mostly made from chemically-grown corn and who knows how it's processed) isn't anywhere near as good as the fresh stuff.

Unfortunately, as you point out, much of our corn consumption comes from secondary sources and who knows how that changes things. And corn-fed animals have very different fatty acid profiles from pastured animals, which affects human health considerably. Then there is HFCS which is the junkiest of junk foods and shouldn't count as corn in any way aside from allergy considerations.

I love one of Michael Pollan's statements which is that ALL traditional diets are healthy ones. Forget the you must eat X and avoid Y. It's all about context. If you ate lots of (organic, hand-processed, heirloom) corn in the context of a traditional diet (which means balancing it with other traditional foods), your liver would be fine (barring pre-existing medical problems which probably arose from a bad diet).

Where we run into problems is with the comodity argument. Corn is corn is corn. It's not. The form of corn, how it's grown, its heritage, and what you do with it all matter.