You really want to go down that rabbit hole? Let me stretch my example just a little bit more then
This "white" kid with a little over 30% black ancestry has a kid with someone who's like one of his parents (you know, the black parent with 62.5% black ancestry and 37.5% white ancestry). Their kid is, by the numbers, about 53% white and 47% black. According to you, this kid is white and not biracial, even though almost half of his ancestry is black?
And again, this is not even taking into account genotype ≠ phenotype, so he/she might look mixed. Or "black". Or "white".
Mind control? It's an observation of how things are. Historically, "mixed" people got lumped in with "black" people and they all predictably mingled. End result, "black" ends up being a broader category than it is elsewhere.