Even submarines powered by diesel engines use, to my very limited knowledge, generators to convert to electricity and then use that electricity for the actual engines. Unlike most cars which do not convert to electricity first. And if the way the electricity was generated is what defines it, then other EVs would need to be allowed as well because the electricity could have been generated from burning fossils.
I have opened a pull request at https://github.com/NixOS/nixpkgs/pull/367042, there might be more changes needed as I had an error in the meta section of the package which I rectified according to the reviewers proposed changes. Not sure this is the end of it, but the request is open