That is completely true, right until we get to the introduction of Korriban in what I believe was either KOTOR 1 or a novel just before, but it was in the early 2000s, so right around the height of the prequel trilogy. Ancient birthplace of the Sith. Once you brought that into the universe, anyone could become a Sith by studying the ancient teachings. Then Ziost came along, and Dromund Kaas (introduced WAAAY before SWTOR). But it all began with Korriban.
True they could never truly be "Sith" in the same sense Darth Sidious or Darth Malgus or Darth Bane was, but as we all know, Exar Kun became a Sith Lord through a simmilar method: Studying ancient Sith teachings and influence from long dead Sith spirits. And yet nobody disputes if HE was ever truly a Sith.
EDIT: I was just going for an IN-Universe explanation.
Woah, I didn't even see your previous post. Great minds think alike, eh?
Though the difference between Exar Kun and say...Darth Caedus is that way too much effort was made in trying to make Jacen all unique and all-powerful and ****** and whatnot. Exar Kun was considered a great character because writers didn't try to beat you over the head with his awesomeness.