It's an illusion. The article even says so, even thought the headline implies otherwise.
It's not really an illusion. Read the whole article and you'll see there's more to it than just an illusion, like improved telecoms.
Superluminal illusion:
Quickly pass a lazer over something very far away (say, the moon). If you didn't know better, one might calculate that a little red dot is sweeping across the lunar surface faster than light! (Of course the dot is not a real unit, but many photons spread over a large distance...)
The phase velocity is not the velocity of light. It's sort of like when you see those old movies of an automobile rolling down the street and it looks like the wheels are moving backwards. They aren't really of course.