They did no such thing. They claimed, and it was true, that they were no longer cooking them in animal fat. All true.
They never claimed the fries were vegetarian, nor did they claim that they were a meat-free product. McDonald's just settled this to avoid bad PR.
What with the flavorings and such used in the food industry, unless something is labelled as "free of animal products," one is rather foolish for assuming otherwise. But McDonald's wasn't going to say that.
The idea of cross contamination from the airborne meat grease is enough to discourage any serious person from assuming that fries do not contain any meat.
SD