I can confirm that there are opensource projects to send and receive (and hence encrypt and decrypt) WhatsApp media files, and they have been around for a long time.

Any forensics expert ought to know about them.

The fact these didn't probably means we should question the rest of the report.

Is there any reason why you can't name them please, for those of us who weren't aware?

The code is a bit nightmarish, but it's hackable to do whatever you like. Whatsapp will ban you if they detect you using it, so make sure to update the client version numbers etc, and use a burner number for testing if using their live servers.