You can object as many times as like, but that doesn’t change the fact that you’re simply wrong.
Do you actually know anything about selection, evolution, or animal breeding? Or is the objection based on just your personal opinion? Because John Mace is quite correct, and you are quite wrong.
How do you know this, exactly?
It would have to be very pervasive and intensive and go on for a long time even to have the effects proposed for it today. (I probably shouldn’t have brought up the example of creating new breeds of animals, because you’ll probably continue to go on about that instead of the more general case.)
Slavery existed in the US for about 250 years, or 10 generations. That’s just not long enough for selective breeding (especially if it were localized and/or intermittent) to create a significant effect in a large population. And any enforced selective breeding was discontinued over 150 years ago, or 6 generations. It’s unlikely for much of a trace of any selective breeding that did occur to persist under free interbreeding among a large population.
I did not post in the other thread because I was waiting for someone to provide a somewhat more factual answer. However, as with any thread that starts with the assumption that certain races are better at sports, it’s important to keep in mind that there are a lot of variables–other than simple athleticism–that could play a role here. Stereotypes change over time when circumstances change. At one point in the first half of the 20th Century, for example, Jewish-Americans were considered to be racially suited to basketball:
Deion Sanders is obviously an even worse example than Jordan, since he played both sports. Mark Hendrickson recently played in both MLB and the NBA, as did Danny Ainge.
The number of pro athletes who COULD have played other sports had they chosen them is a very long one indeed; Tom Glavine, a baseball player, was an NHL prospect. Dave Winfield, a baseball player, was drafted into both the NFL and NBA. A list of NFL QBs drafted into other sports could take a whole thread; Troy Aikman, Daunte Culpepper, Tom Brady, Akili Smith, and others were all MLB draftees. Wayne Gretzky famously excelled at any sport he ever tried. Baseball pitcher Tom Candiotti is one of the greatest bowlers of his time.
I cannot remember precisely where I read it so I can’t link to it, but someone did a study once and found that top success in a sport is strongly correlated with the ability to excel at other sports. If you have two young men of the same age, physical characteristics, statistics and accomplishments, but one if very good at many other sports while the other is only good at the one sport, the young man who excels at many sports is far, far likelier to be a long term success.
And then some goy had to go and invent the jump shot. 
There was similar commentary around that time about how well suited the Irish were to baseball.
The entire point of my comment was to object to your example of creating new breeds of animals, so you’re right that I will continue to go on about that and resist your attempts to frame it as being about something else.
But to the extent that you’re actually disagreeing with me about anything else, perhaps you bring your superior knowledge of “selection, evolution, and animal breeding” to bear and try to explain something instead of declaring things to be so.
Suppose it were the case that the top 10% of a given population for some characteristic (in this case size/strength) had 20% more offspring than average and the bottom 10% had 20% less. Would this not produce any difference at all in this trait, on average, in the next generation (which would then be magnified in future generations)? If not, why not?
As to your final point, you are overestimating the amount of inter-ethnic mating in the US, which is not remotely comparable to “free interbreeding”.
The OP was not asking it is was true, only why someone would think it was true. If a breeding program can turn a wolf into a chihuahua, then it makes sense that one could turn slaves into super athletes. The problem with this hypothesis is not that it is outlandish but rather that it is not true.
First, it would take a massive amount of coordination between slave owners to only breed the strong ones and the average number of slaves per household was six. Secondly, there was not enough time to do so since people can only have so many kids and since generations are spaced out so far. Third and most importantly, it would be economically crazy to do so, since slaves were so valuable any owner would want as many as he could get and consciously restricting the supply by selective breeding would be a massive waste of money that would only pay off generations later.
Another reason we can be sure it did not happen is there is no record of it happening. Freed slaves wrote books about their experiences and gave numerous interviews. I have read dozens of slave narratives and nothing like a systematic breeding program is ever mentioned.
The reason people might think this is true is that it is obvious that black americans dominate most forms of athletics and africans did not do especially well until recently. Thus the hypothesis that what separates black americans from africans is something that happened under slavery.
I wouldn’t go quite that far. Per Wikipedia:
That said, it does appear that the historical consensus is against this type of thing having been consistent or widespread.
Chris Rock using the idea in a setup for a joke during one of his stand-up specials is probably one of the bigger reasons this myth is as widespread as it has been in recent years…
I’m not sure what you’re objecting to since I (and others) said as much in my other posts in this thread and the original GD thread.
Maybe one reason people think it’s true is because some people perpetuate the false claim that “black americans dominate most forms of athletics”. They may dominate some, but not most.
Are you sure that it requires longer than 10 generations?
I remember hearing about this experiment, and how surprised the researcher were at the rapidity of the response to selection pressure.