I remember Ben not being a Sith but I don’t remember anything that explicitly said the Sith were extinct. Maybe it’s a case of Ben wanting to use the name “Sith” to bring a Return of the Sith.
He said “The Sith, the Jedi, the Rebels, let it all die.” He doesn’t want anything to do with them. Palpatine was the last Sith Lord, considering that they have the Rule of Two. A Master and an Apprentice. Vader and Palpatine were the last. We’ll see what’ll happen in The Rise of Skywalker with good ol’ Sheev.