^^^ There was a 2005 issue if SW insider (had 4 different covers--mine was the Obey Vader cover) that actually first said that of all sources. However, that had been put into canon recently; before that uncle george had come up with it as a name.
My guess: It was probably decided that the title itself needed an origin within the time line. The irony is that if you read the novelizations of the movies, you will see that the whole time, the sith (debatably) were as far from achieving ture immortality as anyone could get in their quest for it.
We'll murder them all, amid laughter and merriment...except for the few we take home to experiment!
"I cant see S***! --YOU GO TO HELL!" --Tourettes guy