Your company has been knowingly in breach of the GDPR since it went into effect and shows no signs of stopping despite multiple adverse rulings and fines. There is no reason to trust a company whose entire business model is based on breaching privacy regulations and lying about it.
What you're saying is either an outright lie you are telling us, or a lie you have been told and are yourself choosing to believe to feel better about working for them, or to not put yourself at risk by digging deeper and finding adverse information (which once you know it you may be required to blow the whistle or find yourself legally complicit in the matter).
Basic economics suggests there is no reason Meta (then-Facebook) would pay what it had paid for WhatsApp for "just" a messaging app, especially before it was as entrenched as it is now (and so could trivially be dethroned by Facebook's own offering). They did so because unlike Facebook, people trusted WhatsApp with access to their contacts and that information is extremely important for Meta.
Whatsapp has been audited several times, and to say that an engineer at meta “has been lied to” is laughable given he/she has access to the entire codebase
Source: also worked at meta and had full access to WhatsApps codebase, like most engineers at meta
A conspiracy involving thousands of people lying and no one speaking up over many years (it's been over 9 years since the acquisition) is incredibly unrealistic. Your hypothesis that WhatsApp shares the addressbook with Meta is not at all probable.
What you're saying is either an outright lie you are telling us, or a lie you have been told and are yourself choosing to believe to feel better about working for them, or to not put yourself at risk by digging deeper and finding adverse information (which once you know it you may be required to blow the whistle or find yourself legally complicit in the matter).
Basic economics suggests there is no reason Meta (then-Facebook) would pay what it had paid for WhatsApp for "just" a messaging app, especially before it was as entrenched as it is now (and so could trivially be dethroned by Facebook's own offering). They did so because unlike Facebook, people trusted WhatsApp with access to their contacts and that information is extremely important for Meta.