I don't know, but again: does that matter? Is Amazon obliged to keep their communications with Parler private? Certainly Amazon's customers don't adhere to that norm.

The controversy here seems very simple. When Amazon alerted Parler to specific violent posts, did Parler immediately remove them, or did it wait to remove them until shit blew up? I don't know that I need to care whether or when Amazon alerted BuzzFeed. Amazon says it's removing Parler because of Parler's behavior, not because of something Buzzfeed reported.