nix is a package manager, that is going to install packages in its own way.
the discussion here is about bedrock linux, that answers a specific question: what if, for any reason, you need packages from two (or more?) specific, very different distros?
what if you need packages from BOTH ubuntu and rhel?
to that question, the answer is not "install nix".
which is really bad because i see the value in nix, it's a very nice system, but nix fanboys are getting really annoying with mentioning nix in any discussion where package management is even tangentially related. it kinda reminds me of that old video about mongodb being webscale.
Nix is highly relevant because it solves that case: you can run Nix on any stable distribution like Debian, and the Nix package repository contains a very large amount of mostly up-to-date software, similar to the Arch+AUR repositories.
Another problem it solves more generally is getting access to a large repository of maintained packages from a base system that only has a small repository, even if those packages are up-to-date.
Both of these are reasons you might use Bedrock, so it doesn't seem off-topic.
With nix you can install packages with different versions, you can also install old packages by going back in nixpkgs history and building the package.
Nix being source based means you can install any software package as long as you have access to it's source.
Ofcourse some of this is possible with other systems too but the nix ecosystem makes it far easier to build packages in different languages.
In short nix can treat the disease (missing different versions of packages in distros) instead of treating the symptoms (installing packages from different distros)
