From the article:
The suggestion that the speed of light can change is based on data collected by UNSW astronomer John Webb, who posed a conundrum when he found that light from a distant quasar, a star-like object, had absorbed the wrong type of photons from interstellar clouds on its 12 billion year journey to earth. The "wrong type of photons"? I suppose this is the result of sloppy journalism, so common in accounts like this. But there must be more to it than we're told in this article.
Has to be the wrong wavelength of photons. That's all there is for types.