But that is later. In the 1990's the terms were used as if they were the same thing. or as if Licensing did not allow the use of the term Sith.

Darth Vader was already known as the dark lord of the sith and in Zahn he is referred to as a dark jedi, that is frankly stupid.

Not that i am in any way suggesting they go back and retcon stuff for continuities sake like making Luke a Sith in Dark Empire, or changing the reborn emperors name to Sidious, or redoing tales of the jedi so that Exar Kun's saber is the same colors as Maul's.