It’s a little weird for this article to focus on qsort, as surely the key point is that the algorithm doesn’t have to be stated at all, just the requirements --

It’s a little weird for this article to focus on qsort, as surely the key point is that the algorithm doesn’t have to be stated at all, just the requirements -- that the output is sorted. This doesn’t necessarily invalidate the argument, but it means all the concrete examples seem rather beside the point.