The Nature of Palpatine

[GeneralTacticus]

Not exactly. When Mace accuses him of being a Sith Lord, his reply is:

Am I? Even if true, that's hardly a crime. My philosophical outlook is a personal matter. In fact - the last time I read the Constitution, anyway - we have very strict laws against this type of persecution. So I ask you again: what is my alleged crime? How do you expect to justify your mutiny before the Senate? Or do you intend to arrest the Senate as well?

He doesn't outright deny it, but he doesn't admit it either; he just says it's of no importance.

[Noble Ire]

What about the Stover record when the Jedi attacked him in RotS? It seemed pretty obvious that he admitted to being Sith, or at least belonging to Sith teachings.

It was well known that Palpatine had a facination with the Sith "religion"; he had heaps of Sith artifacts in the waiting room of his office, after all. I imagine that it wouldn't have taken many people by surprise if he claimed to follow their beliefs (if anyone in the public at large even knew what they really were anymore). There's quite a step between that (which, IIRC, is all that the altered recording implies) and actually being a Force-wielding Sith Lord. Only the closest of his advisors knew about that; even the governor of an Imperial world like Bakura thought that the very suggestion that Palpatine was an actual Sith was laughable (TaB).