The two webpages you cited discuss by-product meal, not by-products in general.
Cats don't eat "by-products" in the wild. By-products are not simply organs, or parts of the animal that humans don't eat. They are defined by much more than that. They can and do include not only organs or other parts of animals, but rendered meats (aka animals that have died, sick, roadkill, or otherwise), meats that have been unrefrigerated or improperly handled or stored.
There are VERY loose standards for what is a by-product and what is allowed in pet foods. So don't assume that by-products are just heads and feet.
https://www.dogfoodadvisor.com/choosing-dog-food/animal-by-products/
http://www.dogsnaturallymagazine.com/rendered-products-in-pet-food/
http://www.thecatsite.com/a/by-products-in-cat-food-5-facts-you-need-to-know