How does Palpatine know so much about the Sith anyway?

That's one of the memes going around about the Prequels, often said by fellow Prequel fans in jest.  But it is sometimes presented as a legitimate criticism "how can Anakin be so stupid" and so I felt like writing something about it.

First this opera scene needs to be understood in the context that Palpatine is someone Anakin currently trusts, who's been a father figure to him since he was ten.

Second, Palpatine does not present it as something only a Sith would know, he calls it "The Tragedy of Darth Plagueis the Wise" as if the Shakespeare of this galaxy had written a play about it (I was older then Anakin was in this scene when I first learned Shakespeare had done more then just the five plays everyone knows).

He says it's because it's a Sith legend that the Jedi wouldn't tell it, in a way that implies they do know it.  He's playing on Anakin's insecurities that Obi-Won and the Council are actually holding him back, (a feeling established in the prior film and exasperated just before this when they don't give him the rank of Master), by implying they're keeping this information from him.

He's almost saying it as if Anakin is the only person who hasn't already heard the story.

The problem is the audience had already heard before the film came out that Darth Plagueis was Sidious's direct mentor, and since the Sith were presumed extinct for a thousand years we know this isn't a story that's common knowledge, or learn-able via a simple search of those exhaustive Jedi archives.

But Anakin doesn't know what we know.  And it probably is true that the Jedi don't tell Sith legends, even the ones that should be well known throughout the Galaxy.  Palpatine could have asked about the Tragedy of Darth Malak or Darth Traya or whoever your favorite KOTOR era Sith Lord is and Anakin would have been just as clueless.  Frankly I don't think they even tell the story of Darth Revan given how in ROTJ both Yoda and Obi-Won seem to think the idea of turning back from the Darkside is impossible.  So if he doesn't know any Sith Legends there is no reason to be surprised by Palpatine mentioning one he hadn't heard before.

As a side note, going off the films alone it's not really confirmed Plagueis was Sidious's master, yes his mischievous grin as he talks about the Apprentice killing him in his sleep heavily implies it, but that could be read as him simply having done the same kind of thing, since presumably this is how most Sith Masters got that promotion.

So it's another alleged Prequel plot-hole based on not really paying attention to what's going on.