Looks like they have no idea how to get their software working using Nix. The following blurb is absurd to most GUIX or Nix users:
NOTE: In most cases, end users should never compile fwupd from scratch; it’s a complicated project with dozens of dependencies (and as many configuration options) and there’s just too many things that can go wrong.
Users should just have fwupd installed and updated by their distro, managed and tested by the package maintainer. The distribution will have also done some testing with how fwupd interacts with other software on your system, for instance using GNOME Software.