K8s isn't the only container orchestration platform out there, it's just what is the widely used flavor right now. Any of the micro clusters would still be better than the Docker tools, for a multitude of reasons, and if someone is learning about this right now, they might as well put the effort forth to get familiar.

I've never seen a large scale Compose or Swarm cluster, and wouldn't be working for a team who ran such things. Alternatives would also be: ECS (if on AWS), Openshift, Rancher, and most other cloud platforms have some form of their own that handles provisioning, as well is IAM/RBAC seamless integrations, and other networking integrations for whatever platform.