I looked it up and I found this, "For the experiment, the researchers manipulated a vapour of laser-irradiated atoms that boost the speed of light waves causing a pulse that shoots through the vapour about 300 times faster than it would take the pulse to go the same distance in a vacuum. "
According to that, they used an artificial vapor that had the opposite effect glass has on light. When light travels through glass, it is slower. When it traveled through this vapor, it is faster.
Now here is the thing, this is an artificial vapor, highly unlikely to exist out in the middle of space and even if it did, it would have to be everywhere and scientists have never found any evidence of that.