Darth Krayt was trained by XoXaan as a Sith. Although really, all you need to do is follow the Sith ideology and you can fully be considered a Sith.
Factually, Lumiya, Darth Caedus, Darth Krayt, and the hundreds of other One Sith are all considered apart of the same Sith Order as Darth Vader.
Not to mention the fact that Palpatine, the ultimate Sith, was cloned. The fact he might have no longer followed the Rule of Two doesn't make him not a Sith.
https://www.youtube.com/watch?v=xgBpoiC8qGA
The requirements for the Chosen One prophecy are specifically bringing balance to the Force by destroying the Sith.
The Sith still existing after the Chosen One is dead therefore means the Chosen One prophecy was flawed.
Introducing the Knights of Ren, which both use the light and the dark side (although predominately the dark) and are separate from the Sith Order, was the only way to go to honor the preexisting saga.