Speed of light constant challenged

I was kinda stunned to see this headline: French Scientists: Speed of Light not a constant. This is shaking a foundation pinning of modern science. Even the meter is defined by it. Ok, it is saying "vacuum isn't really vacuum", which I think most scientists can agree on. But if there is a vaccum, the speed of light is constant, ok?

Now, it is blazing fast of course, at 3x10^8 meters per second that is. During that sophomore year in high school I asked the teacher... if it is SO fast, how do we know this value? Can't measure speed with simple v = d/t, right? The teacher smiled, "It isn't so easy"... and pointed me to a location of the textbook containing the octagon mirror experiment. See here for some glorious details.

In science, we often give the ending first. Like we tell students speed of light runs at that speed without telling the story of how people originally thought light runs infinitely fast then calculated that value... We tell students the earth weighs 6x10^24 kg, but often don't tell them how to get this value (by calculating gravity with Newton's formula, google it to find out how).

A science book can be written like a journey of discovery... start with what you see: the sun and the moon and the stars, how I wonder what you are... don't give the modern view yet! Then tell how each piece of knowledge is discovered... point out some old wrong theories (and try not to laugh too hard). Unfortunately I know too little science to write this.

For the record, I never wonder what the stars are like the author of "twinkle twinkle little star". I don't get to see them. I live in a city with too much light pollution.