I wouldn't say completely orthogonal. E.g. the points I've cited are overlap between the two, and ultimately both are meant to host some kind of services. But yes NixOS by itself manages a single machine (although combined with terraform it can become very convenient to also manage a fleet of NixOS machines). Kubernetes manages services on a cluster, but given how powerful a single machine can be I do think that many of those clusters could also just be one beefy server (and maybe a second one with some fail over mechanism, if needed).

If the cluster is indeed necessary though, I think NixOS can be a great base to stand up a Kubernetes cluster on top of.