Errr white people have been the ones saying they bred blacks like cattle for decades. Who else did you hear it from because clearly you didn't read that from any where else. You just contradicted yourself.
The shyt was debunked due to the lack of systematic proof NO ONE can produce, despite slavery being very well documented. Slave masters in the south weren't secretive about their practices, yet you won't find any master diaries or documents indicating systematic selective breeding.
It makes no logical sense at all. If you could breed hundreds of slaves, why would you need to import any? Do you know how cattle works? or breeding dogs? or selective breeding in general? You don't bring in more animals if you're 'selective breeding'.