It’s astounding that a line about a sith Lord being sent by the emperor was never thrown back into any of the Special Editions.
That was my first thought as well. Due to the quality, it’s hard to tell if this is the actual dialogue from the movie (obviously, I don’t mean Vader), but most of the film was redubbed/ADR’ed, so I’m not sure. It’s possible that the line doesn’t exist in good enough form.
It also seems that much of the unused film had not been combed through until Gareth recently started hunting for things, so it’s possible this was under the radar for years?