Ok, I see what you mean now.
What the post is trying to do there is motivate the creation of a new installer, including to the existing Nix community. The snippet you've highlighted is essentially correct, but I still wouldn't characterize Nix as particularly invasive.
The only that Nix strictly needs is to be plugged into your shell. That's it. It doesn't need deep or special hooks into a system just to function.
But including the daemon enables sandboxing for builds that Nix performs, which improves both the security and isolation of those builds, and it also lets Nix be shared nicely between unprivileged users on multiuser systems. For those reasons, daemonful installs are the default and with them come the system users.
(Adding system users is pretty much bog standard stuff for Unix system software, since the main kind of security boundary designed into that system is boundaries between users. Indeed, that's exactly what that's used for with Nix, too.)
The two things I described above comprise the totality of what is required to enable all of Nix's functionality. Everything else that the Determinate Nix installer does as of now is to work around or avoid macOS quirks, and is totally unnecessary for using Nix on any other OS.
The 'root volume' stuff is the result of a collision between the historical and conventional location of the Nix store at `/nix` and Apple's later imposition of a read-only root partition. So Nix installers do a little Apple-specific dance that creates a kind of filesystem volume that doesn't take up any real space or involve any physical partitioning of the disk when they run on macOS.
The other thing this installer does is build in an attempt to self-repair the damage that Apple inflicts upon Nix's sole real requirement by having macOS unconditionally clobber the shell config files under /etc during major macOS updates.
That's it. That's an exhaustive list of all the things a Nix installer does and why. It's not particularly tricky, or hard to remember or figure out. It's not even hard to undo manually— before the Determinate Nix installer existed, I sometimes uninstalled Nix by hand while manually testing the macOS bootstrap scripts for my dotfiles. It was annoying to do, and the uninstallation functionality of the Determinate Nix installer is extremely reliable and convenient and nice. But anyone who knows what `$PATH` is and has ever run `man` before could completely uninstall Nix even if some joker walked over to their machine and deleted the uninstaller.
At the same time, none of the changes Nix installers make on your system affect the behavior of outside programs at all, except by exposing what you choose to install via Nix through standard Unix environment variables like PATH.
Lacking things like kernel components, automatic self-updates, or the requirement for privileged APIs (e.g., on macOS, the endpoint security APIs and accessibility APIs), Nix is not only far less invasive than any endpoint security software, monitoring software, or MDM software you are likely to run on a work machine, but I'd argue tons of common desktop software like Zoom, Discord, DisplayLink and tons of popular macOS powertools like Amphetamine, SteerMouse, SoundSource, etc.
Plus the uninstall procedure with the DetSys installer and its forks is totally conventional and leaves nothing behind: run uninstaller, thing gone.
Nix on macOS is admittedly not an installer-free, drag-and-drop app bundle like some lovely applications get to be. But at most workplaces it's not likely to crack the top 10 most invasive applications installed on the average developer machine, either. Nix installers are just very up front about the things they do set up.
All that said, there are reasonable people who find having a daemon at all offensive. People who are deeply committed to minimalism or simplicity might prefer a single-user install or to use some other tool. But I think for most people, Nix is imo more than fine in terms of invasiveness.