is that you can on occasion be overtaken by a woman, old person, Mike Hock, or as I observed today, a child in school shoes waiting for his jogging hobbyist mother to finish her 3k. They won't be able to maintain the pace for long, but that isn't the point - they have overtaken.
The beauty of fartleks is you can jog along until somebody tries to pass, at which point you can speed up to whatever velocity is necessary.