Having a CLI doesn't mean they are close. In Google cloud, you still work with VMs, cluster, schedulers. In Hyper, you work only with Docker, everything is container native!
Per-second is perfect for Serverless, Data mining, CI/CD, etc. It is simply not cost effective to go with per hour/minute rate.
The idea of Swarm is the cluster works like one single virtual host. In that case, checkout hyper.sh. You don't care about the cluster thing at all in it, because their entire cloud works like one host.
Check out hyper.sh. They allow to launch a Docker image in ~3-5s, without the need to maintain a VM cluster. You can use an API service to automate the provisioning, and then forget about the infra thing.
I don't think rkt is a valid candidate. People are already used to the basic of Docker command. There is no reason to change to something else. If rkt decides to reproduce the feature set, how is that different from a fork?