He needed it.
He already has another 2 escape abilities which is too much for just one champ and it's frustrating as hell.
Also, if Grohk got his Ghost Walk nerfed Zhin can too, and let's not forget that Grohk is a healer who needs to survive more.
And even after the Billow nerf its base duration is still more than Grohk (1.5s, Grohk's 1s), and you can increase it with cards, but they also buffed his healing rate during it with the other talent so it's more of a balance than a nerf unless, of course, you're not using that talent.