No, WhatsApp is not part of Signal. That's false. The opposite is true: Signal is part of Whats App. The problem is we have no idea what else is.
Like, prove to me Facebook isn't logging and storing every single cryptographic key generated in that entire protocol. Oh, you can't because the entire app is closed source? And you just blindly trust that Facebook is doing the right thing and not decrypting every message to scour it for every bit of information possible?
lol.