Just checked out the NixOps manual https://nixos.org/nixops/manual/ and it seems able to deploy to NixOS machines on any cloud or LAN platform. But i also do not find any mention of it being able to deploy non-NixOS servers.
The question is why would you want to have non-nixos servers in that case. If the goal is to have super-fast automation, you still need to unify your setup around something common. I see more benefits in unifying around NixOS than in unifying around Ansible/Pyinfra, as the latter require me to specify my infra in terms of low-level OS- and distro-specific package managers, that would eventually define the same lack of "OS agnostic portability".
it supporting apt doesn't mean it "relies on apt". Looking at the list of options, it supports multiple package managers, including ones for Windows and MacOS.
but then, if one needs to use low-level primitives representing specific package managers, it's hardly aligned with a claim of "super fast infrastructure automation". With Nix I have exactly one package manager for any target platform.
https://github.com/NixOS/nixops