Note that in the video they are playing that thing left-handed.
For someone used to playing guitar/violin/shamisen/etc, the hand that moves along the neck to get different notes is (almost) always the left hand--while the right is used to pluck, handle a bow, and so on. This is right-handed playing, I guess since it's the right hand's actions that drive or cause the strings to play.
For left-handed guitar players, the instrument (and strings) are flipped to the other side--the right hand is fretting notes, and it's the left hand that is plucking/playing notes.