Farron something many people seem to be missing here... Unless I'm misreading, this all happened on a non-GrapheneOS phone. So, the possibility should be no surprise to anyone that Google (being a privileged system component), used its access to your microphone to listen in on a conversation, then sell/share said data with partners (i.e. Amazon).
Again, maybe I'm misreading your post. But if not, no one should be surprised here. I don't care what permissions you did/didn't revoke, Google has more control over your phone than you do, hence why we're all on GrapheneOS, to regain some control.