[YouWhy]

To the extent a random person's evidence on the Internet amounts to proof:

From people at Facebook circa 2018, I know that end user privacy was addressed at multiple checkpoints -- onboarding, the UI of all systems that could theoretically access PII, war stories about senior people being fired due to them marginally misunderstanding the policy, etc.

Note that these friends did not belong to WhatsApp, which was at that time a rather separate suborg.