“That, then, is the true essence of a Sigma. It is a man who deliberately stays out of society’s hierarchies, and goes and creates his own familial hierarchy. And that is a worthy goal toward which most men should aim themselves.”

Speculation, and nothing more:
Feudal/Aristocratic societies tended to reward strong family structures in the near term. There is strength in high trust and resource dedicated operations.
They also reward strong family structures over successive generations. The near-term benefits only grow as time passes, and the family becomes a clan becomes a tribe becomes a barony or nation. Genesis is rife with these developments within the Hebrew patriarchs but also their neighbors (Lot’s daughters sprout Moab, Esau’s become Edom, Nimrod births Ninevah, etc).

Did the sigma have a stronger role in human civilization before our representative and collective governance? Is the Sigma ascendant in the consciousness again as we run headlong from republic to oligarchy to aristocracy?

At best that is a tertiary explanation for the n-Gram search data, but it fits my confirmation bias, so it must be true 😉

Precisely