There was a thread where @TLATER mentioned this a while ago, albeit with the .xz archives instead.
At the time, I’d made a PR on the Nix templates repo (which I see you’ve found) Move channels to tarballs by peterc-s · Pull Request #100 · NixOS/templates · GitHub
I might get round to swapping it over to the .tar.zst archives instead but the PR was incomplete and no maintainer has commented.