it says on one of the loading screens "sith of this era are not a species but followers of an ideal, true sith died out centuries ago" so the sith are just darkside force users with the strongest leading, so revan and malak were just dark jedi, with revan being the most powerful in the universe, therefore leading the sith
<{POST_SNAPBACK}>
As you said the Sith are followers of an ideal, they follow this ideal, therefore they are sith!