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.
Well the agent did beat Jadus or Zhorrid so anything can happen.