Explained far better than I would:
Now you have your answer, this is not a process that has gone on thousands of years, we're talking a couple decades.
Actually McDonalds doesn't use pink slime anymore. So what parts of the animal is McDonalds using that are undigestible? This was the topic.
Also that article did not say those parts were undigestible, just that they were being treated with amonia as they come from parts of the animal likely to come into contact with cow fecal matter.
However, like I said before we have been using those parts of the animal for thousands of years. Traditinoal dishes like Sausage in Europe commonly use parts of the intestines, that are contaminated and not fit for human consumption prior to processing. Tripe is a common dish in many parts of the world. Cow rectum specifically can be found in many parts of Asia as a "delicacy".
So the issue isn't really with the fact that some parts of the cow are "indigestible". The issue is with how the meat is being prepared. In the case of pink slime, we are talking about the use of Ammonia to sterlize. However, once again, McDonalds doesn't use pink slime. So what's your point? and what does this have to do with a debate about the 100% label on McDonalds food.
And honestly, if you are so concerned about eating cow feces, you should avoid pretty much all food. Cow feces is a common component of most fertilizers, especially organic ones. Organic fruits and vegetables will be covered in it.