[jpgvm]

Nix generally speaking has a global "nixpkgs" version (I'm greatly over-simplifying here ofc) in which there is a single version of each package.

This is likely the source of their commit based versioning complaint/issue, i.e the commits in question are probably https://github.com/NixOS/nixpkgs versions if they aren't maintaining their own overlay of derivations.

This is in contrast to systems that allow all of the versions to move independently of each other.

i.e in the Nix world you don't just update one package, you move atomically to a new set of package versions. You can have full control over this by using your own derivations to customise the exact set of versions, in practice most folk using Nix aren't deep enough in it for that though.