The capability they were claiming to have would make a three letter agency very excited. If they truely had the ability to listen to your microphone, transparently without notifying the user, they could sell that tech to every regime that wants to snoop on people, for millions of dollars.
Instead they claim to be using it for Ad-tech, where if it existed, would make it trivial to discover and flag as malware.
Apple and Google would also be very keen to find and squash whatever loophole let’s them record without showing the notification.
Its just an extraordinary claim, which if true would have been exposed/validated by security researchers long ago.