After Qui-Gon Jinn reveals his suspicions, Jedi Master Ki-Adi-Mundi declares to the Council, "Impossible! The Sith have been extinct for a millennium."
"The Acolyte" showrunner Leslye Headland has talked a lot about wanting to show the fall of the Jedi, and what could be more poetic than 100 years of lies told by Ki-Adi-Mundi.
Ki-Adi-Mundi’s investigation of Jedi murders in Vernestra Rwoh's small task force could suggest something sinister, like the Jedi Master is part of a century-long cover-up.
Perhaps Master Mundi knew about the Sith threat and dealt with it, and Rwoh's team, already failing to inform the council, decided to simply keep quiet.
There are always two Sith: a master and an apprentice so they don’t wipe each other out, and to keep them hidden from the Jedi until they enact their plan for revenge.
If Mae’s ally is the master, they may have successfully covered up the existence of the Sith — or Mae's master is an apprentice for an older, wiser Sith, hiding in the shadows.