I know someone who in fact did sign the NDA for this and they said they would never have dreamed of saying/posting ANYTHING about the product or even mentioning the NDA or that they even signed one. They didn't say they weren't allowed to say anything about it but they did say that it is HIGHLY frowned upon to do something like this. Legal or not, it's a good way to burn bridges, not just at Apple but throughout Silicon Valley, I think.
Also, I've actually met this guy. First, this article calling him an engineer isn't accurate. He was a designer on the webpage side, I believe. It's not like he was involved at all on the headphones project. And his tweeting about it is definitely making the chatter around with his old coworkers in a not great way. Kinda sounds like he just wanted to boast that he knew about it, then realized it was a dumb thing to do.