An Empire ruled by a Sith Lord was an empire ruled by a Sith Lord, it was however not necessarily a Sith Empire. This was due to the complexity of the definition "Sith Empire". Well, a Sith Empire is an Empire ruled by a Sith Lord, but technically a Sith Lord could choose to not name his empire as a Sith Empire. The main reason for doing this was because the name "Sith Empire" usually scares the crap out of people, some Sith Lords did not want to scare people, they just wanted to kill them. And Killing people is much easier when people don't run away when they hear your name, or when they see you.