I feel quite confident based on first hand knowledge of code, system design, and the many, many privacy reviews we had to go through when building new features to ensure we didn't accidentally log or otherwise infer data we weren't supposed to.

WhatsApp architecture is designed with the assumption that the server could be compromised and yet such an event should not result in any message contents being revealed. Furthermore, the encryption function is designed to ratchet and rotate keys so that a leak of a key at a given point in time would not compromise past and future messages.

So yes, I have a strong sense of confidence that message contents are not exposed to Meta and, given the bar set by privacy reviews, I don't think Meta would do some backdoor workaround like scraping the contents off the device and sending an unencrypted copy. To be clear, my claims are specifically around message contents and when it comes to certain metadata (ex. the sender/receiver, the names of groups, etc) I don't recall the exact details of how they are treated.

Now, despite the fact that I've said all this and that my knowledge on the matter is fairly recent, I'm not sure I could ever say anything with absolute confidence. The code base is huge and not open source. I obviously have not seen every line of code and as you pointed out, there's always a chance some company policy changes happened without my awareness. So I would say "highly" confident but not "absolutely" confident.