Should the unthinkable happen, and someone “breaks out” of docker jail, they’ll only be running in the context of the user running the docker daemon on the physical host.
There is no daemon in rootless mode. Instead of a daemon running containers in client/server mode you have regular user processes running containers using fork/exec. Not running as root is part and parcel of this approach and it’s a good thing, but the main motivator was not “what if someone breaks out of the container” (which doesn’t necessarily mean they’d get all the privileges of the running user on the host and anyway it would require a kernel exploit, which is a pretty tall order). There are many benefits to making running containers as easy as running any kind of process on a Linux host. And it also enabled some cool new features like the ability to run only partial layers of a container, or nested containers.