I am unfortunately not able to reproduce that (with the same hash for nixos-wsl.tar.gz, I am however already on WSL 2.4.5.0). Usually when WSL just prints its help text, that means that there's some sort of error with the syntax of the command you ran, not with a distro. I copy-pasted your command (just replaced the name, because NixOS is already taken on my system) and that run through just fine:

󰍲  nzbr@PULSAR
PS T:\> sha256sum.exe .\nixos-wsl.tar.gz
\576f2dda94b0dde278cd9ebbdf04b53bd9c174fba5dd4e98bd9af227dd97fc24 *.\\nixos-wsl.tar.gz

󰍲  nzbr@PULSAR
PS T:\> wsl --import nix-dbg $env:USERPROFILE\NixOS\ nixos-wsl.tar.gz --version 2
Der Vorgang wurde erfolgreich beendet.

As this seems to be a general WSL problem to me, rather than a NixOS one, you could try opening an issue in microsoft/WSL. Maybe someone there knows what might be the problem here

Figured it out and it's a subtle syntax issue. So this command does not work:

wsl --import NixOS $env:USERPROFILE\NixOS\ nixos-wsl.tar.gz --version 2

But this one does.

wsl --import NixOS $env:USERPROFILE\NixOS nixos-wsl.tar.gz --version 2

It's the leading backslash that causes wsl to fail. I'll do a quick online PR to fix it.

Originally I thought this was an issue with my name containing a space, but nope after removing the leading backslash it works fine. No surrounding with quotes needed either if I'm using $env:USERPROFILE.