Impedance of headphones
Impedance does vary by frequency. However, for many things it doesn't vary very much by frequency within the expected operating frequency of the equipment, and a single number is much easier to handle than a frequency response graph. So people quote a single average number.

In fact, your linked article admits this: 


  Headphone impedance can vary by a factor of 10 or more over the audio band. The SuperFi 5 is rated at 21 ohms but varies from 10 ohms to 90 ohms. 


That's quite a lot of variation! It also explains why this causes variation in the audio quality when driven from different sources.