This stupid name, "True Sith" is causing much confusion. Kreia used the word true to separate "TS" from Sith Empire's military forces and robots, that are too called as sith.
Original sith would've be much better name IMO.
Basically, there is only two possible versions of true sith.
Unlikely one: They are developed from those Dark jedi that were defeated at the end of hundred year-darkness, but did not end into Korriban and Ziost, instead they fell (with their space ships) on elsewhere in unkown regions (Known space was much smaller then, Korriban & Ziost were both parts of UR ) and created an empire there. However, I am quite sure that it is stated that all surviving dark jedi's ended up to Korriban and Ziost, and if that would not be true - which is damn unlikely - then they should not be called sith, since these dark jedi would never meet sith species and become their lords.
Likely one: As I am too lazy to rewrite (and fix typos!) I just give link to topic where is my speculation on what TS are. Link