Honestly, Darlene, that has a bit of a "snake oil" smell to it, if you don't mind me saying so. Where does he get this stuff, and why does anybody listen to him?
When you inherit a certain segment from a common gt-gt-gt grandparent, for example, the odds of even getting a match at all with someone in your own generation (a 4th cousin) is something like a 50/50 proposition. Then you manage to get a match from yet another 4th cousin - through sheer "force of numbers" - because it was just a very prolific family, or just from dumb luck.
And so this guy is trying to tell us that it doesn't mean anything, because the three of us might also have some OTHER unknown common ancestor, for whom the odds of getting that kind of match is even smaller - MUCH, MUCH smaller?
Doesn't it strike you as convenient, that this theory is completely unprovable - that practically nobody has a complete tree going back to all their 8th-gt grandparents, or wherever?
What is this guy trying to accomplish? Establishing a name for himself by throwing stones at accepted practices - the old "see how smart I am" scam - when what he's saying can only be idle speculation?
"Distinct possibility"? What, exactly, is THAT supposed to mean? Well, I'll tell you what it means. It means "I don't know what the probability is, and if somebody tried to calculate it, and gave you a number, you'd all laugh me off of this stage."
There's going to be cases where endogamy is a real issue, and the tools of genetic genealogy might be rendered somewhat useless - when your ancestry includes multiple ancestors who were from families that lived in close proximity where there was a small population, and other special cases. At least, useless for that part of your lineage. These are exceptional cases - and not hard to spot.
But what does he imagine anybody is going to do with this "profound wisdom"? Throw the proverbial baby out with the bath water, out of extreme paranoia? Is he even making some sort of specific recommendation?