See here: Multi-User Mode - Nix Reference Manual
Technically you could do a single user installation, but this is off the beaten path so you’ll probably run into lots of problems doing that. I think most installers don’t support that anymore.
Your login manager is supposed to hide these users; their UID range is supposed to be in a block that the system considers “system” users (read the comment, follow the links, Arch will do something equivalent): nixpkgs/nixos/modules/misc/ids.nix at 2e1715cf7cf3c1e79436d566962aeedaffbfb49d · NixOS/nixpkgs · GitHub
Arch is probably just not set up to consider that UID range “system” users. You’ll have to consult the arch wiki on how to change that, it might be in your login manager’s documentation, or perhaps it’s supposed to be done with kernel settings; I don’t know the details of your setup.