I also read this same quote from one of my slp classes. I have not met any African Americans that stutter, instead I have met a couple of middle eastern people who do stutter. I am from the Caribbean but stuttering already exists in my family. I think people that grow up in low socioeconomic status don't have the same opportunity to get early intervention when they are young so that's why maybe you see more A.A or Caribbean people that stutter. I didn't receive therapy at a young age due to the lack of funds and mainly ignorance.