I understand and you make good points. However, the claim is that EVERY single human is at least a 50th cousin to each other. All it takes is a single case to disprove that so the burden of evidence is on the people making that claim. I completely understand it as a typical statement but not as an absolute claim. You can get some strange anomalies when you compare extremely disparate populations. If the North Sentinelese really are as isolated as most anthropologists think they are, they are greater than 50th cousins to almost everyone else in the world. If they had influence from other related groups, they aren’t demonstrating it because their language is not mutually intelligible to the closet islanders or any other in the world for that matter.
It is hard to believe that is possible in the 21st century but they aren’t faking it as far as anyone knows. They use stone-age weapons, they don’t use fire and they don’t have clothing. It is difficult for anyone to even photograph them from the most sophisticated surveillance aircraft because they stay hidden under the dense foliage most of the time but they will attack with spears and bows and arrows if anyone lands on their beaches or tries to fly over at low altitude.
Again, I believe the math works for the claim in general but maybe not in the absolute sense. There are still some really odd anomalies out there that probably defy it.