Not all individual strands of hair will be exactly and perfectly cylindrical. That's an image using an electron microscope, which has VASTLY superior resolution that the simple visible-light methods used. Hair is extremely elastic, so there's no reason to assume it isn't being deformed by tension. Which we know for a fact it is, because it's tied in a knot. Why would the photographer lie about using hair, and use something else instead? What motivation exists to be honest about a grain of sand, but dishonest about the hair?