That's not exactly true. Authentication by Face ID confirms the person who set up Face ID has unlocked WhatsApp, not necessarily the owner of the iPhone.
Ah, but it's incredibly unlikely that someone else's face is used to unlock an iPhone, isn't it? Yes, so it's more likely that it is the phone owner, right? Okay, you're with me. So, without Face ID authentication, is it more or less likely that the person who simply launches and uses WhatsApp is the owner of the phone? More likely? Right. So what exactly are Facebook getting from knowing the phone's owner authenticated that they don't already get from someone just using WhatsApp as before?