By uploading their code to a public GitHub repository, they agree to GitHub's terms of service[1], which grants other users additional rights to this code.
Although NaturalMotion Limited says they grant no rights under this license, you are allowed to fork this repository, as they have implicitly granted those rights to you by agreeing to the GitHub terms of service.
"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."
Let's assume for a second that viewing and forking are clear and obvious copyright rights (they aren't), and that they somehow cover downloading and using (they probably wouldnt).
"By uploading their code to a public GitHub repository, they agree to GitHub's terms of service[1], which grants other users additional rights to this code.
"
This has literally never been upheld in court, nor is it likely to be upheld in a way that people want (see the above parts about what rights it really covers), to be frank :)
Additionally, it would be inapplicable in this case.
You can't give away rights you don't have.
Note that NaturalMotion didn't upload this, Justin did.
If you upload some Google owned source code, you haven't suddenly granted rights on behalf of Google, you've just likely committed copyright infringement.
Someone relying on the github license grant (ie a viewer or downloader) would still be a copyright infringer, and in turn try to use the "innocent infringement" defense (17 USC 504(c)(2)). They would fall neatly into the category of people who believed they had a license to use it. The problem, of course,is that this defense is eliminated completely by the presence of the notice.
See 17 USC 401(d).
So the people who downloaded and used it would be screwed as well, despite whatever github has written here :)
(I have no evidence or position on whether the result would be different if naturalmotion knew or acquiesced explicitly to the uploading to github. But then you'd still hit the same viewing and forking rights are not the same as downloading and using rights, and it's highly unlikely you would convince a court they are the same).
I'm no lawyer but I'm not sure that allowing others the ability to view and fork your repositories is the equivalent of granting them a real license to the code. I don't think the definition of fork implies much in the way of licensing, but I could be wrong.
I'm thinking that they are talking about forking within the GitHub infrastructure, which means having the same code in multiple locations within GitHub (as a copy). The licensing doesn't change when the code is forked.
I'm not saying that you can distribute the code or compiled code outside of GitHub. As I understand it (IANAL), it seems like you have the right to edit this repository on GitHub as long as it stays on GitHub and not redistribute it through any other avenues.
Edit: Someone mentioned that Justin uploaded this repository, and not NaturalMotion. If Justin doesn't already have the right to distribute this on GitHub, then the rights granted to you by the ToS probably don't apply.
Although NaturalMotion Limited says they grant no rights under this license, you are allowed to fork this repository, as they have implicitly granted those rights to you by agreeing to the GitHub terms of service.
[1] https://help.github.com/articles/github-terms-of-service/
§ F. Copyright and Content Ownership
"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."