There are lots of native NixOS tools for managing whole clusters (NixOps, Disnix, Colmena, deploy-rs, Morph, krops, Bento, ...). Lots of people deploy whole fleets of NixOS servers or clusters for specific applications without resorting to Kubernetes. (Kube integrations are also popular, though.) Some of those solutions are very old, too.

Disnix has been around for a long time, probably since before you ever heard of NixOS.