Asking to create an account later makes sense, the lessons are open to try without creating an account here http://www.tryfoundation.io/learn/. We wanted users to create accounts in order for them to save progress and code especially for further bigger projects, thoughts?
Visitors that know that they don't know enough will always signup. I knew that I'd like to learn more Foundation, so immediately submitted my email address. But I would suggest that a decent slice of your users don't know what they don't know - ie. that there is a Bootstrap alternative that is just as flexible, powerful, and easy to learn. Getting those people three steps down the path before giving them a hurdle of signup would, in my opinion, result in a larger user count and greater project impact in the long run.
The structure of these lessons is great - probably more useful than Bootstrap 'getting started' pages. The use of JSFiddle style learning is awesome.
The only thing I don't like is that it requires email/password - however given it's not built by Zurb I can see why the makers have gone down that path. Perhaps a better method would have been to only ask for those details once a user wants to go beyond the first few lessons?