That's not what that document says. iMessage has a design flaw (which is pretty obvious if you think about how it works) that allows it to theoretically be backdoored. In other words, they made a trade-off between usability and security, and (in my opinion, and clearly yours) fucked up. That's very different from saying that they deliberately built a backdoor into the system, and I think that some of the things they've done (like explicitly noting when someone else has been added to your iCloud account, and will be able to decrypt upcoming iMessage messages) goes to some length to mitigate those issues, and make clear that the existing design is more incompetence than malice.
That said, all Apple would have to do to fix this is to allow advanced users to see all keys listed as authorized for their account. I'm getting increasingly annoyed Apple hasn't done that.
Nowhere in the article does it say that Apple actually compromised the end to end nature of an iMessage conversation. All I see is this:
> Apple could collaborate with law enforcement to provide a false key, thereby intercepting a specific user’s messages, and the user would be none the wiser.
Key word is "could". Apple "could" also use its signing keys to install any kind of software on your phone to do whatever it wants. For example, to read your keychain and pull your private keys.
And due to the design of CALEA (it dates back to 1994) they can't force Apple to do this easier, which is one of the things that started the entire encryption backdoor debate in the first place.
That said, all Apple would have to do to fix this is to allow advanced users to see all keys listed as authorized for their account. I'm getting increasingly annoyed Apple hasn't done that.