It’s a name that was created for that storyline. Palpatine, much like the term Sith, was used for the OT and its marketing materials. Sidious was treated as his truer name, with Palpatine being more of the disguise.
Every other film in the franchise, including first instalments of two other trilogies. This wasn’t like Lord of the Rings, where it’s one story spread over 6-9hrs. It was a first story, but he failed to tell a complete story.
Was the word “Sith” ever uttered in the original trilogy, though? (excluding early drafts, deleted scenes and the EU material). Can’t remember if it did.
Sith was never said in the OT. Nor was Palpatine or Sidious. In the EU, sure, but not the actual movies. Remember when the emperor’s name was Cos Dashit?
Ewok was never said either, for that matter.
And, for all intents, “Darth” was a first name until Phantom Menace where it became a title. Phantom actually was a massive lore dump for the Sith in general.