The "Creative Commons
Attribution-NonCommercial-NoDerivs 3.0 Unported License" is an "open source" license. The source code itself is "open" and can be redistributed for free (beer not speech). But it is disappointing that they have opted to prohibit derivatives, and it makes their posting of the code to GitHub an interesting choice.
A pull request would technically be violating the license, no?
IANAL, but as I understand the license, yes it would violate the license. Although, if I released something under a no-derivatives license on github, I'd be less worried about pull requests and more worried about derivatives made outside GitHub.
What if you clone the repo before the license was added? Then do you get code in the public domain?
No, the default state of software is copyrighted and not redistributable. You need a specific license to give you the right to modify and/or redistribute software (though I think copyright should be changed to allow private modification for certain purposes).
The source is clearly open. I'm looking at it right now.
That it doesn't match Stallmans definition of "free" doesn't mean that it's not open source.