Because the girl wasn't moving! Double Duh!
Answer this similar question: If you are a dog, how can you type your reply, because dogs don't have fingers? I particularly invite your attention to the words, "because dog's don't have fingers."
Well, I would be in a more receptive mode of the photographer's explanation if he would have provided the photo EXIF along with his explanation.
I dunno, the depth of field is fairly deep, and lighting pretty good indicating to me that his explanation doesn't necessarily ring true. (just my opinion). Look at the consistent width of the "blurred" microphone cable. Oscillation of only a portion of a cable of this length would have some sort of periodicity or oscillation, and not display a consistent horizontal displacement as the photogrophers explantion would imply.
I will agree that if this is a result of post processing, it sure is a careless and ham handed technique.
"Because the girl wasn't moving! Double Duh!"
The photographer said she was. Are you contradicting him?