The source is in the article linked in the Tweet. I'm not sure why this post links to a Tweet but not directly to the article: https://www.theverge.com/2021/10/22/22740701/microsoft-dotne...
I glanced through the article - it mostly deals with dotnet watch fuckup. This article is 2 weeks old, nothing was known about Nat's decision to leave at that time.
Edited for clarity.