From what Obsidian said in press releases, their idea for the "true Sith" was a faction completely unconnected to the Sith Order founded by the Dark Jedi of the Second Great Schism who mingled with the Sith species. It was supposed to have been a group that built their empire tens of thousands of years ago.
This is just personal conjecture, but I do believe that there were going to have something to do with Adas and his legion of Sith warriors who defeated the Rakata, took their space vessels and conquered other worlds circa 28,000 BBY. I've posted my theories on the matter on these boards several times, and if I come across any of them, I'll be sure to link you to them.
In other news, I just finished the novel last night. I have to say that I quite liked the direction that Karpyshyn took. My only complaints are Revan's name and how he identifies himself, as I said before (after all, why did he not recall his original name when all of his memories came flooding back near the end of the book?), and the fact that
Revan dons his mask as a light sider. I've never liked this idea. To me, Revan's mask and familiar costume are identified with his time spent as the Dark Lord of the Sith. During the Mandalorian wars, they signified his continual fall from grace. They don't belong on a redeemed Revan, in my opinion.