The article mentions word "set" only in the statement of the problem. And even in that statement it's made clear that the choice of words is intentional, becaus

The article mentions word "set" only in the statement of the problem. And even in that statement it's made clear that the choice of words is intentional, because the underlying data structure is not specified, so it doesn't have to be an array immediately. Ordered collection != sorted collection, btw. Sorted collections are actually more like sets than like arrays. A data structure that stores the elements in a sorted fashion is actually able to store an unordered collection with naturally defined insertion / deletion operations. (In OCaml, Set data structure is represented as a balanced binary tree, for instance, and it requires an order relation defined for its elements.)