It was me
Aside from Lewandowski, most of Bayern's attackers looked quite poor under Ancelotti last year. People overreacted to him having a bad year in a team that underperformed (as far as their standards go), and never seemed to mesh well with Ancelotti's tactics.
It's night and day between this year and last for Costa. And this year he is even more impressive than under Guardiola because he's developed into an all-around player, showing responsibility and hard work tracking back and defending that he never was required to do under Poop.