That comment about their security updates is a couple of months old. Zen has made significant changes to how they handle releases to stay up to date with Firefox, some of which were mentioned in the thread.

The Nix package is functional. Nixpkgs sometimes chooses not to add beta software that is receiving very frequent updates, especially if the package takes a lot of resources to build. In those cases, it makes more sense for the package to be maintained outside of Nixpkgs for the time being. NixOS users can get Zen from this Flake, which patches the official binary. COSMIC Desktop is another example of this.