Those were the dark ages because previously there was a highly developed civilization with its sophisticated culture, art, science, technology and liberties, and then the western world sunk into an era where almost all of it was temporarily forsaken. It fell into the darkness and chaos (at least from the point of people who lived in the Roman empire). Barbaric tribes raided the lands, many local wars again started since there was no central rule anymore, Catholic church imposed insanely strict rules - pushing art and science knowledge and skills of humanity hundred of years back. Ideas of health and sanitation and hygiene that Romans cared a lot about were practically non-existent, so diseases ravaged the population. And then came the plague and killed more than half of the population and the rest mostly fled to country-side leaving many fortified cities completely uninhabited. It doesn't mean that people were stupid or that they didn't craft pretty things back then, of course they did and the conquering tribes also had their primitive culture and crafts, but it was very limited and heavily sanctioned by church, unlike before, and almost all art was reduced to just religious themes or simple decorations. In Byzantine Empire there was even a brief period of iconoclasm, when it was forbidden to show any humans in paintings, like in Islam. So, yes, maybe it wasn't the world from Mad Max movies and there was no Orcs, but it wasn't that far away from it as the author makes it sound. Just compare it to the renaissance (which was still lagging behind classic era) and you'll see how dark it really was.