@snoke123
The Supernova ability is unique to the star crystals. And the author who wrote that story said that "no jedi or sith is that powerful" when asked if a force user could cause a supernova. My interpretation is that he thought of the corsair as a "holy grail" kind of thing, some impossibly powerful object that allows the characters, for the sake of the plot, to do things which have never been and will never again be done.
As far as in-universe explainations are concerned, I'd argue that since sith alchemy has been shown to be somewhat of a science, with learnable rules and complicated recipies, that Sidious would be able to do it by creating a ritual which had never been created before.
So yeah, if you want to believe, I guess you can, but story-wise, the supernova ability is very "non expanded universe," in the sense that it exists solely for the purpose of one story and doesn't extend outside of that to other ones. For good reason, too.
Oh, and yes, of course Sidious knows alchemy. He's done it more in the comics and books but one of the later episodes of TCW did show him performing a ritual to create a "dark illusion" through the force bond between Yoda and Dooku.