I guess the author has a different definition of "especially" than most. To most people, the observation that about 12% of the population is responsible for around half of the violent felonies committed in America would qualify as "especially."
That 12% rate is deceptive. When you eliminate the elderly, children (ie real children, not teen gang-bangers), and, for the most part, women, the violence-prone black portion of the US population is far less than 12%.
Break that down further. Black males between 15-29 make up 3-4% of the population.