No, that’s relying on github… it’s using the registry to resolve, which actually adds a second point that would fail if github dies - because the global registry lives on github, and the registry entry’s url points at github too. It’s also objectively worse if you have a local registry entry that points at a local path, because then your flake’s lockfile will be unusable for anyone else. So, no, stick to the github:NixOS/nixpkgs/<branch> format.
And never base your system on the master branch, use the nixos-unstable or nixos-xx.yy branch.
Anyway, we don’t need imagine what a mirror looks like because a university mirror already existed a year prior to this topic’s creation: