I think it goes a step further. Sidious figured out what Plagueis couldn't, he learned to cheat death for himself. I think Sidous died at some point and Plagueis brought him back to life. Having actually been brought back to life gave Sidious the insight to learning how to cheat death for himself later. In fact to kill Plaguis, Sidious may have had to kill both himself and his master. That's a move Plageuis wouldn't have suspected. (It's also possible while Sidious can save his own life, he never learned how to bring others back from the dead like Plagueis did.) What Sidous does is different from the Jedi. Obi-Wan and Yoda live on as one in the Force and their bodies vanish. That's like transcending death. What Sidious does is cheat death and on some level it's like he didn't die. The point is Sidious is still mortal. If someone knows what he is doing, they could prevent him cheating death again to stop him once and forever.