>"Could youn please define "closed"? As far as I understand, WhatsApp is based on The Signal Protocol."
It can be based on anything they like. But if you are running a binary blob on your phone, you have no idea what the actual code is doing ALL the time. And you certainly don't know what their servers are doing. That is "closed".
>"Also, there are no known remote vulnerabilities to The Signal Protocol (that I'm aware of). The FBI has broken encryption on the protocol but this was done physically. This is unlike the remote access that's described in the article."
The encryption can be rock solid and unbreakable. But if their app will send them the keys if requested in some manner, then you are done.
>"Also, do you have any evidence"
Nope. I have no idea. Like I said, it is unlikely there are any shenanigans going on. But it is plausible.
>"or a conceptual idea of how WhatsApp would have "master keys present at the start"?
Yes, that is easy. Your local machine creates the private key it is going to use and the app transmits that to their servers and it is stored. Or, requested later under certain communication and it is sent at that point. Do you know their code doesn't have that ability? How would you know? Especially if it never does it unless requested in some secret way....