Author here. It's mainly for k8s resources; but if you install operators like external-dns or something like crossplane into your cluster, you can manage infra too.
I guess the point is: what if you don't have a cluster.
There is life before (and beyond) Kubernetes.
I currently have a custom script that is a mix between terraform and ansible that sets up a proxmox cluster, then a k3s cluster and a few haproxys with keepalived on top. Granted, maybe not the most standard setup.
But then with images I'm on the hook for patching... Not simply auto-patching via apt...
But that would complicate synchronization with resources outside of k8s, like tailscale, DNS, managed databases, cloud storage (S3 compatible) - and even mapping k8s ingress to load_balancer and external DNS.
So far I feel that everything in terraform is the most simple and reasonable solution - mostly because everything can be handled by a single tool and language.
Unfortunately, I'm not a big fan of the yaml-hell that crossplane is either.
But as a Terraform replacement systeminit.com is still the strongest looking contender.
Consider CDK8s (Typescript or Go) or Jsonnet. We evaluated Cue and the two aforementioned options and ended up with CDK8s using Typescript and it's incredibly powerful.
There's nothing like terraform plan/apply?
I mean - some help wrangling yaml is welcome - but I already get (some) help from terraform with the k8s provider there...
Do you check in the generated yaml in git, or just the typescript code?