The article's about statistical averages, and their implications for the modern social sciences, not about historical classes and castes.
Indo-European society probably had three castes: medicine men (responsible for law and justice, good relations with the gods, and magic; "wizard-priests" would also work as a description of their role, but feels too flattering for such an early, primitive society); warriors; and commoners. India split commoners into two classes (artisans etc., and laborers); Europe and Persia grouped skilled and unskilled commoners together. Europe and India both saw intense competition between the two top estates -- roughly speaking, magi and knights -- to determine who would rule society; in Persia, the magi more or less won without a fight (and sacralized the knights), thanks to Zoroaster.
In short, it wasn't nearly as simple as royal versus non-royal, at least not in the Indo-European world -- and the high-ranking estates had obligations as well as privileges.
Indo-European society probably had three castes: medicine men (responsible for law and justice, good relations with the gods, and magic; "wizard-priests" would also work as a description of their role, but feels too flattering for such an early, primitive society); warriors; and commoners. India split commoners into two classes (artisans etc., and laborers); Europe and Persia grouped skilled and unskilled commoners together. Europe and India both saw intense competition between the two top estates -- roughly speaking, magi and knights -- to determine who would rule society; in Persia, the magi more or less won without a fight (and sacralized the knights), thanks to Zoroaster.
In short, it wasn't nearly as simple as royal versus non-royal, at least not in the Indo-European world -- and the high-ranking estates had obligations as well as privileges.