This makes no sense. There is nothing to prove that that WhatsApp is really using the aformentioned Signal code. It's closed source, so it could be anything inside.
Your objection to a perfectly cromulent answer is just namecalling. People take apart and find vulnerabilities, including cryptographic ones in closed source software all the time.
It's a closed source app. Translation: you and I don't know what it does. Take it apart all you want, but you're not going to find any backdoors or learn about how good the E2EE implementation is. Claiming otherwise is ridiculous.
It's a closed source app. Translation: you and I don't know what it does.
This just isn't true. It's a claim trivially disproved in public by, say, gazillions of detailed P0 posts. Again, all you have is namecalling and confidently stated things with obvious counter-examples.
Claiming otherwise is ridiculous.
Yours is an extraordinary claim that requires, never mind extraordinary, any evidence.
This is my point exactly. It doesn't matter as long as the app is closed source. You could do the worlds most secure E2EE implementation, but then send a copy of all keys to Facebook servers.
It's trivially easy to encrypt the traffic and then send a copy of the private key to Facebook's servers. You would not be able to decrypt it, but they would.
Why is that? With WhatsApp client being closed source, we simply don't know if it really is doing E2EE at all.