Screen temperature is a public health feature which should be implemented by the OS. Until this feature arrives in iOS, the next best option is for apps to implement it. The ebook reader Marvin and Koala web browser both have controls for color temperature.
It's NOT open source as per the readme: "I purposely have not added an open source license to the project and am intentionally retaining copyright for the code, meaning that it may not be redistributed (in code or compiled form) without my permission."
The author states: "You may think it's dumb to have open sourced it then [...] even if it's under copyright. [..] So basically, it's open source so people can learn from it.
So this is public source code which is under copyright.
AIUI most FLOSS code is under copyright [0] the distinction being that the author(s) grant others a licence to use, copy and distribute under certain restrictions.
It seems that the author here wants people to be able to read the code, but not distribute it, primarily because he does not wish to tread on the toes of f.lux' developers.
[0] In fact, AFAICT unless a creator specifically and explicitly waives such, all work is copyright, because Berne treaty.