The wrong assumption is in BuzzFeed's reference to "99% of child exploitation." The letter, which they deserve credit for quoting in full, says that "more than 99% of the content Facebook takes action against – both for child sexual exploitation and terrorism – is identified by Facebook's safety systems" (which rely on Facebook being able to automatically analyse unencrypted messages), "rather than by reports from users" (which could still be submitted if Facebook implemented E2EE by default).