Not in all elos. Riot balances the game around 4 divisional skill bands. Rek'sai has historically been high elo skewed. U.GG shows a 1.16% increase for Rek'sai's win rate in Elite play (D2+). Though admittedly Lolalytics has them much closer together (Rek'sai still in the lead in Elite play). Not saying Nunu couldnt be tapped down, but the stats arent 1:1. They are probably nerfing her early game in order to reduce her high elo skew.
Yeah it's that. Rek'Sai is absurd in 0.1%+