That certainly looks less trivial than writing it directly in your .gitconfig file.
It's the same condition and setting as what's in the article, but now with a build/templating stage and a new programming language to learn with unusual syntax.
Curiosity got the better of me so I looked it up at https://nix-community.github.io/home-manager/ and it indeed does purport to provide benefits I guessed at and then some.
Whether that's better than just manually managing things yourself is altogether a different matter.
In TFA the author must set up two configurations: the .gitconfig, and the file which is included in the .gitconfig. Home-manager does this automatically through one config parameter. That is what I was pleased with and wanted to share.