no its not the passive
its his very high earlygame damage
provided he lands his abilities he will outdamage both opponents combined in addition to that he has the E spam to poke
and the Q to zone/disengage
and at 6 you can't peel off his engage as a support unless you want to be stunned for 4 seconds yourself in which case you probably just lose that fight anyway.
The passive does help early on, it prevents both of you dying because you can stand and fight after you die allowing your carry to continue fighting or disengaging
fighting sions passive early on is the dumbest thing you can do ever