I assume "Sith cultists" aren't cultists who are Sith, but a bunch of Sith fankids, like the Church of the Force (the villagers at the start of Force Awakens*), but for the dark side. Their existence wouldn't have any bearing on the Rule of Two and wouldn't necessarily have any link to the actual Sith lineage. Though maybe one of the purple dudes from the second Death Star (From this sick album cover: https://i.imgur.com/jhjoFwc.jpg) was a Sith cultist, who dragged Palpatine's corpse/spirit back to Exegol.
But I haven't read whatever book this info comes from.
(*I know, J.J. didn't explain any of this either... Now I'm wondering if Lor San Tekka's "Without the Jedi, there can be no balance in the Force" prediction was successfully addressed)