I found that to be a really good read. I have to say though, none of the design decisions read like mistakes if you consider that perhaps the encryption is ment to be reversible by WeChat.
They call the fact that WeChat generates the client encryption keys server side "highly unusual", instead of the obvious, which is they found an easy way to keep prying eyes out of their protocol, while still giving themselves complete decryption control.
First thought: Oh hey it's Nutty Putty Cave guy! Second thought: Holy shit I need to gtfo the Internet.