Or jedi are still not very common and/or famous, so lightsabers are just seen as some old antiquated weapons. Thus, the stormtrooper may not assume that the one wielding it has magical forces that makes him a superior combatant.
That would explain the confident stance.
Then again Kylo Ren, or the Knights of Ren in general, may have equipped them with those, whatever they are, weapons.
For all we know they might have killed several jedi before, or they may never have seen one before like you said.