My thinking is that in ancient times, the Sith openly ruled the galaxy and used non-Force-sensitives as slaves, and that the people we see in ROS are descendents of people who bought into the claim that the Sith are the rightful rulers of the galaxy who the Sith hid on Exegol after open Sith rule of the galaxy ended.
I wish they had the stadium audience be all the pre-Sidious Sith.