Oh, it's very simple. Home-manager is something not yet finalized, optional and unofficial. Flakes was added in Nix 2.4, but you don't have to use it all.
You don't want to add unnecessary complications. You can do without home-manager and flakes on a single-user machine. This is just one of the system configuration options. I have tried deploying the system with flakes/home-manager, done it with docker and even with nix-env
. The performance gain is not noticeable.
Home manager and flakes is something optional in today's NixOS. I don't see any obvious use for a single-user home system. Of course, many people go the flakes-setup route. NixOS has many customization options, up to and including docker installation.