It’s also not always about intentional inbreeding.

Many people don’t know who their biological parents are. While not a significant issue with adoption, if a guy has multiple kids who don’t know their siblings in the same area. Then they are likely geographically close and of similar ages...

“Johnny be Fair” https://mainlynorfolk.info/folk/songs/johnnybefair.html

And defines this to mean double-first-cousins and closer (perhaps this is standard). Also, as far as I could skim, this is the prevalence today, rather than an estimate of long-run prevalence.

They must surely have data on simple first cousins, but it didn't jump out at me.

For traits, the effect on non-double first cousins can be linearly approximated, which (as far as I know) is a good approximation for inbreeding depression.

For example, taking Fluid Intelligence Score (basically how smart you are), in Table 3, its extrapolated effect size is -3.43, so:

First cousin (relationship coefficient 0.125, inbreeding coefficient 0.0625) would give -0.214 to FIS. To put that in context, for medication, that would be considered a small effect size. So inbreeding at that level is a bit like taking a stupid pill.

Simple first cousins have a coefficient of relationship of 0.125, which is below the threshold considered in the paper.

Received wisdom is that a coefficient below 0.25 doesn't cause serious deleterious effect. I have no idea if that's well supported, but I imagine it's relevant to why it wasn't studied.

The motivating force for this definition is uncles/aunts and closer; it just so happens that double-first-cousins have the same genetic relatedness as double first cousins.