Yes, there really was a "Dark Age" in Europe after the demise of the Western Roman Empire. But your article is purposefully confusing what we call the late Medieval era (11th century and after, when civilization reappeared), and before. And it is ignoring (purposefully again) many facts.
This decline didn't deal only with economy and science, but with demographics. In just a few centuries, the total population of Italy, France, or Britain was divided by two or three. Thus, pretending that the German tribes did bring "freedom and peace" is a very dubious claim. War was everywhere, everytime, it was a lawless era of confusion with no centralized governments -or very weak ones-. The introduction of Christianism, the intolerance and bigotry of early Christian sects almost totally destroyed the ancient world. But after a few centuries, Western Europe eventually recovered, and yes, the late medieval era was probably more free than Roman times, and saw astounding progresses in economy, engineering, agriculture, and demography despite large pandemics (the black death being the worst).