US Federal employees don't have copyright on their work. That statement in license.md refers to how it is clearer how to get to a permissive license via the contract-theory mechanism, GPLv3 is more reliant on copyright as the mechanism of providing its protection of software freedom. This is still a work-in-progress and needs public/community review.
From license.md on their github
> We hope to also support GPLv3 for a project, but the weirdness of not having copyright makes that a little more difficult.
Are they meaning strong copyleft here? I thought maintaining and using copyright law to enforce copyleft provisions was a key idea of the GPL?