There appears to be some pretty extreme racism in there masked by it being entirely about white people. 'This end of the country is made of accredited professional stock, therefore is genetically smarter vs this bit is inbred cousin-marriers who are just gonna be dummies.'
I wondered for a while if it was using genetics to attempt to map where cultural divides had reduced intermarriage, but yeesh. It's hardly acknowledging non-genetic cultural patterns or historic concentrations of wealth at all. Kinda hard to climb the regional intelligence ladder (barf) when your school system isn't getting the big city bucks and there's not really been a big national push to patch up district and state funding disparities.
Well he also mentions southern Europeans being "clannish" vs the "outbred" NE residents. What he's probably talking about here is the difference between more urbanized population vs rural population family models. Southeast England was much more urbanized and densely populated, so they probably had smaller family sizes in terms of extended family and more contacts to select for marriage. Different dynamics completely, especially once the industrial revolution really kicked off.
It's common for two extended clan-type families to exchange female members by marriage over multiple generations. So any thing about "marrying your cousin" needs to be understood in terms of this family model. You have two allied clans, and over generations, they intermarry daughters from one clan into the other. Note: under this model that daughter now becomes a member of her new clan, and you generally cannot marry someone from the same clan. So yes there is inbreeding, but it's controlled through ritual and customs. The inbreeding is because two clans can swap members each generation, not because you marry someone from the same clan (although a clan can split into branches, and it's possible to marry someone on a different branch. But technically, they're a different extended family). This system may have largely broken down however in more recent years. Think Beverley Hillbillies "Granny" character. She's in there to represent the
clannish extended-family thing, which was seen as being distinctive to those Appalachian peoples. Granny exists as a character because to a urbanite Los Angeles person with their nuclear families and small households, the idea of grandma living with the adult son and his family was seen as unusual and different.
Even American indian tribes basically worked the same way, I was reading a book once that was talking about anthropology explaining totems. Originally they were thought to have some mysterious ritual significance, until someone explain the very sensible and explainable reason for having them, at least as far as some tribes went. Totems were basically clans within a tribe, and they dictated who you could marry, so if you were e.g. in the Wolf totem, you couldn't marry someone from the Wolf totem, but you could marry someone from the Eagle totem. The wife then takes on the husband's totem. Then next generation, your child could marry someone from the other totem again, who would technically be a cousin. So this, and the clan system of the Irish and Scottish who settled Appalachia were both methods for
controlling inbreeding. They weren't sources of inbreeding.