They were disciples of the Sith teachings from the Trayus Academy, and went to great lengths to restore Korriban and explore the ancient history of the Sith. They are as much the inheritors of the Sith title as Vitiate and his cronies.
Now, I have a question. (This is related to the actual Sith-y-ness of Malak.)
While it is known that Revan and Malak were pawns of the Emporer, the question rises: Are Revan and Malak true Sith or just Dark Jedi? While they were under the Emporer's influence, they didn't seem to have complete training in Sith ways, and merely used Dark Side powers. So, is Revan and Malak truly Sith?
I personally dislike this whole "annointed by the Sith Emperor" bit. Perhaps Malak saw it as a license to wreak havoc, but Revan clearly turned around and immediately started preparing to forge his own empire to crush Vitiate.