The article doesnt say 15mm lower than GTI it just says lowered 15mm.
It obviously means over car it is based on.
Every review I have ever read has something in it like this:
"The only thing I have noticed recently is quite how hard the ride is and things can get a little bouncy at times. Whereas the slightly stiffer and lower suspension makes the Edition 30 great fun on the right twisties the car is undoubtedly better suited to smoother surfaces."
And my ED30 is DEFINITELY lower than my mates std Gti and is harsher on Peak District B-Roads to boot