I had my doubts, too: it seemed like a very strange way to "leak" the response, ...

I had my doubts, too: it seemed like a very strange way to "leak" the response, and the wording and phrasing weren't perfect. ARS seems to have called a spokesperson who didn't deny it, though:

http://arstechnica.com/tech-policy/2014/06/netflix-refuses-t...

Or at least ARS has a spokesperson saying the "letter speaks for itself," which suggests authenticity.