Does Kha'Zix get +2 Might if I play him in response to an opponent's Rengar during a showdown?

When units are played, they use the chain but are finalized and resolved immediately, so there is no window to react to a unit being played. In this scenario the opponent ends up with two units at the battlefield, so Kha'Zix does not meet his trigger condition and does not get the +2 Might or 2 XP.