The onus is not on us to prove that it's not E2E encrypted, but on Meta/WhatsApp to prove that it is. The only way they can do that is by open-sourcing the client application, and providing a method for anyone to verify that the binary on their device was built from those sources, without modification.

Anything else is just theater. Anyone who is worried that their communications could get them arrested or attacked cannot safely use something like WhatsApp. There is no way to trust that a third party's keys haven't been added to a conversation, or that the client isn't leaking message content through some other means.