The article confused me very much. I had to read the original question and the first part of the article once again, to conclude what the point was. I now think it boils down to this: if pointer p points to one of the elements of array A of size S, then the p >= A && p < A + S will yield true, but if a pointer x points to something outside the allocated region, x >= A && x < A + S may also yield true (or yield false or terminate the program or do anything else, because the comparison invokes undefined behavior).
p >= A && p < A + S
x >= A && x < A + S