Let's not pretend he didn't know what he was doing. There are textbook examples about people who blast off a rant to the entire company and then are somehow surprised when it becomes public (http://www.telegraph.co.uk/news/worldnews/northamerica/usa/1...).
It wasn't even meant for the entire company. He gave it to a small community of people within Google who later distributed it across the company AFAIU from his interview with Jordan Peterson (https://www.youtube.com/watch?v=SEDuVF7kiPU).
He shared it in a forum that any and everyone in the company can access, and is intended to be able to access. That's essentially the same thing. This wasn't a private email chain that was leaked but an intentional dissemination by the engineer to google's employees at large.
