i could be wrong but what i think is being said here is that most of the plots of the OS were cribbed from other literary sources...
jane eyre, frankenstien, rebecca, the turn of the screw, the picture of dorian gray, the telltale heart...all of these stories were rewritten into the DS framework utilizing it's setting and characters but were not exactly "original" stories being told.
the end result felt very fresh and original but the writers heavily referenced other material.
Oh. Well, I'm not interested in holding anything up to some simple, absolute standard of
total originality. The DS storylines that in fact did just lift plots from old novels or films are the ones I don't like that much anyway (Jeckyl/Hyde, Rebecca). There wouldn't be much point in remaking those DS storylines anyway.
The great DS storylines, however, like the original Barnabas story and then his flashback, took basic
elements from older stories (rather than copying plot, characters, and dialogue, and basically doing a remake of something earlier) and reconfigured them into a compelling new situation, then explored new avenues based on it.
All good stories take basic life and plot elements that have been used many times before, and arrange them into a new pattern, creating a fresh situation, and then they insert different characters into it, and see what original direction they can take it in.
This is different from looking back fondly on the whole Barnabas/Josette story, or the story of Barnabas being released, etc, and saying "This story is so great, that we should keep refilming that same story."