I've found the Less Wrong description to be profoundly disappointing, so let's do better.
When you talk about a group for a purpose, the label used for that group should be causally relevant to the purpose.
I am hungry. I have a stick, a carrot, a steak, and a mug. I can carve up this group in various ways.
"Let's eat something natural."
"Let's eat something fleshy."
"Let's eat something long and thin."
Technically, if I have the correct background knowledge, any of those would work. In this example, were I sharing my dinner with you, any of those, while a bit stilted, are perfectly understandable and we could continue a discussion and ultimately sit down and dine.
However, when doing something difficult, we need every advantage we can get. Apparent understandability is not good enough. If I want to eat something, I should start by considering an edible property. We can say, "Whites are outbred," but 'white' is not a relevant property, and leads to interminable arguments about whether slavs or jews are 'white.'
When investigating, it's entirely possible that nobody knows the relevant background information. If I say, "Long thin things are edible," even formally restricting it to a simple set like above, we may end up trying to eat a stick.
Hajnal Europeans are the set living behind the Hajnal line, and thus have a history of manorialism and Catholic social engineering against the clan and in favour of itself. These things have a causal relationship with outbred nuclear families, which has a causal relationship to democracy resistance and lack of corruption and so on. Their colour, like the shape of the fleshy thing above, is coincidental.
Politicians/sophists love doing this wrong, precisely because it leads to confusions which they can exploit to get you to serve their interests instead of your own.