JPL is run by California Institute of Technology and the work they do is under contract to NASA, so I would be cautious about calling things that stem from there a public domain government work.
In this case, the linked software definitely isn't public domain and it's most likely not open source (since this wasn't an approved release through JPL/Caltech). Also, given that the linked software's running on a spacecraft, distributing it might be in violation of federal law (thanks to ITAR/EAR-- spacecraft and related articles are on the US Munitions List).
IME (and I've worked on a number of federally funded projects at this point, both in and out of academia), these contracts typically get written such that the institution retains all rights to the software, but grants the US government unlimited rights to use the software.
(I will note that it's also not unheard of for these contracts to stipulate that the end product becomes open source-- software I've worked on that was funded through DARPA is up on GitHub, for example.)