I could write a book on this, but here is a brief overview of what I think the "true Sith" are (though I doubt this was is what Kreia had in mind):
The Sith (species) that the Exiles came upon and lorded over in 6,900 BBY were said to be a primitive race, knowing little about the cosmos and worshiping natural phenomena as deities (thus why they were so quick to honor the conquering Dark Jedi as gods). But it is said that in 28,000 BBY, Sith king Adas of Korriban defeated the Rakata and relocated his people to the nearby planet Ziost, suggesting that they were accustomed to space travel at the time. How then are they a primitive people by the time the Exiles come around?
I believe that at some point either Adas himself or one of his successors must have observed a drastic difference in the Sith people: some were modernists while other held onto their archaic ways. Believing the latter group to have been weak and unworthy, the modernist Sith abandoned Ziost and the school of Sith who were behind the times, and set off to build an empire elsewhere, eventually coming upon the Unknown Regions and building the Trayus Academy on Malachor V.
So we have a rift in the Sith race: those modernists who set out for the Unknown regions and those who stuck to their old ways. The former group would consider themselves the "true Sith," and the latter group was left to devolve on Ziost, eventually conquered by exiled Dark Jedi who founded the first Sith Empire. All the while the "true Sith" were building an empire of their own in the Unknown Regions, plotting to take over the galaxy when the time was ripe. Revan may have found evidence of their existence during his time as Dark Lord, and these memories may have come flooding back sometime after the events of the first Knights of the Old Republic game. This may have been why he departed for the Unknown Regions: to battle this potential threat to the Republic and the galaxy (or, in continuation the non-canonical dark side ending, to his new Sith Empire).
Originally Posted by a silly noob
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