I don't buy it.
If people want to argue over which of the codex versions is the valid one that is fine, but the idea that the actual layout of the PDF is some kind of mistake is ludicrous.
They actually cropped and pasted artwork over part of the rules for allies in the Daemonhunter codex. Someone had to fire up photoshop and intentionally alter the image of the page. It's not like they just forgot something.
I'm 50/50 on whether the email is a hoax or not. If it isn't then GW must think we're all really stupid. They probably do think that, but then there are a lot of hoaxers out there too. Somebody wrote an entire Blood Angel codex as a hoax and this is just an email that would take ten minutes to type up.