That’s not how encryption works. But you’re not wrong about it being owned by meta being a problem. There’s more info in a message than just the contents.
IIRC whatsapp’s automatic backups are stored in cloud and unencrypted by default. So it takes only one person in the group chat to who has backups enabled (pop up comes every now and then if they’re not) and no password is set (not required, takes effort and will to set) to leak everyone’s messages.