The point is, the more processed an ingredient is, the more rapidly it is digested. Think about how difficult a raw potato is to digest; we cook them to break down the strong plant cell structures and make them edible.
Even thoroughly grinding beef makes it more digestible than a whole cut. Even if you got fancy steak and ground it at home.
I don't know about McDonald's specifically, if they really do only grind their meet and they don't add any artificial ingredients, then that's lovely. I like a quarter pounder!
I think the point is, the large quantities of very easily eaten, very easily digested, very easily available highly processed foods are not good for people. And even if the ingredients sound similar, research is emerging that they're just not quite the same. Or more accurately, they don't have quite the same effect on our bodies as a less processed version of the same meal.
My personal theory is that we're not really designed to cope with the way UPFs digest in our bodies. I once read a study on hunter gather diets, and they actually get quite a large proportion of their calories through simple sugar (honey). But this would go along with lots of vegetable matter and minimally processed meat.
My own experience is that food that is highly digestible, quick and easy to eat and super palatable mainly falls into the UPF category. Even a delicious home made sandwich with homemade bread is not as soft and easy to eat as soft fluffy processed bread and soft chicken breast with added water etc. For some reason these foods do not really fill me up and I could (and do!!) eat sooo much of them.
Even if there is literally nothing else negative about them (which I personally don't believe), I would still move toward limiting UPFs because I they are so so likely to lead to weight gain.