He just said its not a manual penalty.
https://twitter.com/JohnMu/status/1219557678965579776
I don't think he knows whats going on as well. Let alone the fact that he actually had no idea what happened on 17th December just proves this.
Suddenly on 17th December Google started to re-evaluate our websites and the index went to shit...COOL STORY BRO...
EDIT: The thread on Twitter, so many german websites. I had this index/traffic problem on so many of my german websites. Seems like Google.de got hit hard with this.