The problem is they're assuming there's a specific point at which the parent wouldn't be a chicken but it's offspring could be defined as a chicken. But the evolution from it's earlier form would have happened gradually over many generations, there wouldn't be a clear dividing line. You could pick any one bird and say that's the first chicken, but it would still have almost the exact same phyisical characteristics as it's parents, and still obviously be able to mate with non-chickens of the same species as it's parents. So it would still be the same species as it's parents, regardless of whether you defined it as a chicken or not.