If you can read your WhatsApp messages on your phone and you don't control the WhatsApp binary... then Facebook can backdoor WhatsApp to read the decrypted message.
I'm not suggesting that no-one can read them. I have no idea. I am saying that his testimony makes me very comfortable that Facebook is not because he has way more to lose from lying than from not in those circumstances.
I have trouble with this logic. Also, I'm not quite clear - what are the consequences for lying to Congress?