Two things (not exhaustive). First, as the article notes, nixpgs has a lot of packages. Second, nix matches Gentoo's customizability but with good binary caching for common builds. (Kind of; that's very approximate)
Edit: I should amend: nixpkgs is already huge, and with flakes you can trivially get packages from anybody you trust (like Arch's AUR but distributed and easier), which covers even more packages.