When you write code, you automatically have copyright on it (in most countries). Failing to include a copyright notice or including a license text does not make your work fall into the public domain.
As far as I can tell this code is not currently open source. The only permissions given are:
"now fork it, fix it and send pull requests". Which omits some important permissions, without which this cannot be called open source.
The author publicly announced it was open source via HN.
"Open-source software is software whose source code is published and made available to the public, enabling anyone to copy, modify and redistribute the source code without paying royalties or fees.[19]"
http://en.wikipedia.org/wiki/Open_source
"Works are in the public domain if the intellectual property rights have expired,[1] if the intellectual property rights are forfeited,[2] or if they are not covered by intellectual property rights at all."
http://en.wikipedia.org/wiki/Public_domain
He put it on GitHub, and he announced it was Open Source on HN.
"There's no license in this case and you cannot claim any intellectual property of the code. It would be the same if you uploaded the content on your own site without providing any license. According to the terms:
We claim no intellectual property rights over the material you provide to the Service. Your profile and materials uploaded remain yours. However, by setting your pages to be viewed publicly, you agree to allow others to view your Content. By setting your repositories to be viewed publicly, you agree to allow others to view and fork your repositories."
http://stackoverflow.com/questions/4007674/whats-the-default...
As far as I can tell this code is not currently open source. The only permissions given are: "now fork it, fix it and send pull requests". Which omits some important permissions, without which this cannot be called open source.