The issue is the game does a complete crap job at explaining how powerful members of the dark council are. For instance, Thanaton is considered very weak as it takes 2 chapters to get the power to kill him yet a FREAKING SMUGGLER can solo beat several of these guys. While it may seem like that simply suggests that my sorcerer is weak Thanaton was shown to be able to essentially insta-kill me (a force user) whereas more powerful dark council members could barely touch my scoundrel (who cannot use the force assuming legacy is non-canon)
Well, if I remember correctly, the Smuggler can only fight them in a world quest. If the world quests are ever get a canon character who completed them, I doubt the smuggler will get Corellia. (It is also possible that the world arcs are completed by groups in canon. A Smuggler killing Darth Decimus together with a Jedi Master, one of the greatest Jedi fighters, and an elite soldier isn't that unrealistic.)
Still, I think one could say that probably most Dark Councilors are not used to the smugglers fighting style. A Sith leader normally only has to fight other Sith. They may also have some experience with regular soldiers. But the smuggler doesn't even fight like a common criminal, he has his own tricks and tools. So if he has to fight a Sith Lord, it largly depends on luck. This time, the Smuggler was lucky.