Darth Nihilus' Sith identity only demonstrates that titles can be misleading:
"He cares nothing for the Sith or its teachings… or the Jedi. And when the Jedi are dead, he will feed on the galaxy, the Republic, and eventually, consume the Sith as well."
I mean really people have being claiming Vitiate is not a Sith long before he dropped the Sith Emperor title.
That said in so far I've only seen him use the dark side. So what difference does it make?