Zahn used the term Dark Jedi to refer to Palpatine and Vader. I guess he had no idea what a Sith was?
I mean other than wanting The Noghri to be the Sith race, and Vader Lord of the Sith.
Correct. Lucy Autrey Wilson, who was the President of the EU, was Lucas’s secretary when the Original Trilogy was written, so she knew about the early drafts in which Vader is one of many “Sith” and rejected Zahn’s Noghri-as-Sith idea.