It was a stupid comment, which Stones should apologize for. However, I do not think Dwight was truly comparing Africans to dogs it was the extension of a dumb joke he makes about the unknown age of a dog from the pound. However, the comparison is more a dumb joke that you can date the age of a human or dog like a tree versus a direct comparison of an African to a dog. I get the impression that if someone had a super old white grandma that no one knew when she was born he would make the joke "just cut off her legs and count the rings." Given historical characterizations of Africans, I think it is a racially insensitive comment, not intended as being racist.
What he said was "You know my answer to the age old question about how old are these guys really when the run these incredible junior times, and everything else, is the same answer I've always given when we don't know how old a dog is that we've gotten from a, a rescue, just cut off the leg and count the rings. It's just that easy."
It occurs at the 3:34 of the broadcast.