Ansible could theoretically be made to work with NixOS by having it manipulate the /etc/nixos/configuration.nix instead of touching files and starting services

Ansible could theoretically be made to work with NixOS by having it manipulate the /etc/nixos/configuration.nix instead of touching files and starting services directly. But you're still going to run into so many incompatibilities between NixOS and other Linuxes (esp. because of the different filesystem layout) that it's probably not a path worth pursuing. It's always a recipe for disaster to have two configuration management systems competing for power on one system, and NixOS very much is a configuration management system.