Wth.. They didn't buff him. They nerfed his void spikes some
They didn't mash him into a gooey paste after zapping him with an electric fly-swatter either.
Riot has a pretty long history of blasting assassins into Evelynn tier when they have a decent showing in competitive play. Like Evelynn, that was nerffed so hard she got garbage tier nammed after her for a few years.
Rengar was nerffed into no-mans land (until his triple Q was discovered and widely used, then he was nerffed again). LeBlanc was nerffed pretty hard despite having one of the worst win-rates when she did get popular, Kassadin was finally nerffed pretty harshly, Diana fell pretty far from where she started, Yasou just took a pretty big nerf, and Talon even took a noticeable nerf despite never actually being a top-rate pick.
Kha'Zix, Zed, Fizz, and Lee Sin are about the only assassins that have really evaded that thus far. Fizz actually got a buff to compensate for a Lich Bane nerf.
Kinda surprised Fizz hasn't gotten hit that hard, but Zed seems like the poster-child of what Riot wants an assassin to be, and for whatever reason they seem to be committed to tweaking Kha'Zix into a healthy spot with love-tap nerfs rather than overnerffing him.
Kind of didn't expect Kha'Zix to remain relevant up to Worlds, but he maintained being a really solid jungler for the entire season and arguably one of the 3 strongest assassins in the game.