Who is Sidious refering to. By all the implications he's probably referring to Plageus, but is he? \
It's clear in the film that Palpatine is referring to Darth Plageuis the Wise. It was obvious that was Palps' master, and that Palps killed him in his sleep and it was "ironic that (Plageuis) had the power to stop others from dying, but not himself."
I don't think there is any question about this.
You're thinking more about the opera scene than the actual exchange that's being referenced in the first post. And there are three sets of exchanges about this topic that hold significant bearing on a Force user's ability to maintain their identity in the Force.
The first exchange is obviously Palpatine with Anakin at the opera. He tells Anakin that Darth Plagueis was so powerful that he was able to keep others from dying. He goes on to say that he taught his apprentice everything he knew, and then his apprentice killed him in his sleep. And then says that it's ironic. He could save the people around him, but not himself. So if this is in fact how Sidious killed Plagueis, how would Plagueis have been able to prepare himself to become one with the Force if he had been unaware and asleep? It's not plausible.
Then there's the exchange in Palpatine's office. That's where he tells Anakin that to cheat death is a power that only one has achieved. But if we work together, I'm sure we can uncover the secret. So where does that leave what Palpatine previously told Anakin? Apparently Plagueis had not passed on the secrets of Force immortality to Sidious. But Palpatine uses that bait to seduce Anakin. And he never identifies who has managed to maintain their identity in the Force. Should Anakin learn that it was Qui-Gon Jinn, the Jedi who helped free him from slavery, it could be a potential source of conflict for the newly dubbed Vader and turn him away from the Dark Side.
And then there's Yoda's exchange with Obi-Wan. He tells Obi-Wan that he has training for him, and that he will learn to commune one who has returned from the netherworld of the Force. Qui-Gon Jinn.
The intended scene where there is a dialogue between Yoda and Qui-Gon would only have further cemented this. But lo and behold Liam Neeson never even recorded the dialogue and that plot point was left much more understated.