Yeah, unfortunately it does seem that nixos.org has “always” been the home of nix first and NixOS second. To be clear, this is an observation, not an argument against change.
Doing some archive.org spelunking, the first snapshot that I see is from Apr 25, 2008 and starts with:
Nix is a purely functional package manager. It allows multiple versions of a package to be installed side-by-side, ensures that dependency specifications are complete, supports atomic upgrades and rollbacks, allows non-root users to install software, and has many other features. It is the basis of the NixOS Linux distribution, but it can be used equally well under other Unix systems. Read more…
Perhaps this early site was clearer about the differences between Nix/NixOS and it does get “Linux” in there.
You may be interested to know that there is a new docs push and that the website implementation is at https://github.com/NixOS/nixos-homepage.