I'm curious why you think it's handwavy.

I've done this work on other mobile apps (not WhatsApp), and the work is not out of the ordinary.

It's difficult to hide subtleties in decompiled code. And anything that looks hairbally gets special attention, if the calling sites or side effects are interesting.

(edit for edit)

> That's certainly the only way messages could be uploaded to Facebook!

Well, there's a primary pathway which should be very obvious. And if there's a secondary pathway, it's probably for telemetry etc. If there are others, or if it isn't telemetry, you dig deeper.

All secrets are out in the open at that point. There are no black boxes in mobile app code.