I can't see how it could be wrongful termination. Ignoring all of the subject matter, he wrote something unrelated to his job in his free time and posted it in a company mailing list for his coworkers to see. I'm going to fire someone for posting their hobby writing on company mailing lists, sorry. I don't care if it was an essay on how delicious hamburgers are, or Twilight fan-fiction: it is not his job to be sharing that stuff at work, and it is inappropriate to ask your coworkers to read it for you and share their thoughts, when it's not their job to be doing that either. I'm going to go out on a limb and guess that maybe some of the people on that mailing list have actual work to do.

You want to write an essay, fine, but don't bring it to work. You have a complaint about how management works, fine, schedule a meeting with your boss, but don't undermine management and just blast it to a mailing list. If I wrote an email detailing my criticisms of my boss, without talking to them first, and sent it to a company-wide mailing list, you'd better believe I'd expect to be fired.

That said, Google certainly handled it poorly. They didn't respond at all to the initial mailing list posting. They only reacted when people online started talking about it, basically. Plus they misrepresented what he wrote (it sounded like they didn't even read it) when they posted a statement on Twitter.