Are the new Chaos Knights that are now available for pre-order the same as Renegade knights? If so, I don't see the Desecrator and Rampager listed. If not, are there really only two chaos knights?
In 8th yes, Renegade and Chaos Knights are the same thing.
Excluding Forge World, Chaos Knights have 4 different Knight Chassis: Despoiler (Questorus), Tyrant (Dominus), War Dog (Armiger) and Abhorrent - this latter type is the new model with the differently jointed knees and includes the Rampager and Desecrator.
So broadly, compared to the Index, names have changed and there are also two new Knights that are somewhat
unique to Chaos included in the Codex, but there are significantly more than just two Chaos Knights.
Hope that helps?