In all fairness, "loyalty" and "identity" are core moral foundations for "conservative" or "right-wing" politics, and Sailer is often cited as a member of the so-called "alt-right". "Race", i.e. skin color, is a very salient feature which tends to correlate (at least in the United States; we would see very different results in such places as Brazil) with the kind of cultural distinctiveness that tends to create and sustain ingroup biases. So, it's not very surprising that the 'alt-right' would be biased towards this kind of ingroup solidarity.
"Black solidarity" is indeed harder to explain. It may reflect a morally conservative attitude on the part of some blacks; it may be proof that left-wing folks are anything but immune to ingroup bias, at least in some circumstances; or it may be that what we take to be "black solidarity" does not reflect a true ingroup in the moral and cognitive sense, but rather a mere coalition or bloc based on shared interests, which is a fairly common feature in modern politics.
"Race", i.e. skin color
It's not just that. Colin Powell is considered an African American despite being pale-skinned.
If it's worth saying, but not worth its own post, even in Discussion, it goes here.