Is WhatsApp client open source?
Publishing the source online doesn't fundamentally change the way that interested parties can inspect the client. And the reversing process has the nice property of actually guaranteeing that the thing you are looking at is what runs on the device.
Also, WhatsApp's T&C forbid you from doing it anyway.
If the key ends up used for signing and authenticating messages and for nothing else, then it's sort of safe to say that it's not leaked. (Sure there might be some other part of the code that reads it indirectly, but that's also something that will likely not be named leak_priv_key() :))
The only thing that would help is open source + trusted reproducible builds.