I’d understand Han Solo in ANH calling Vader a “dark Jedi” the way my parents might call a Playstation a “Nintendo”, but it doesn’t make sense for Yoda to call someone a “dark Jedi”.
none of the post-ROTJ darkside-users qualified as being part of the Sith order.
This is true. Before TPM was released, EU writers were told not to use the term “Sith” to refer to anyone other than Vader if the story was set within the lifetime of characters from the movies.