I don't think the author intends to provide a serious production solution, at least not for now.
Runit tools in busybox - up to the task?
Let's test busybox's runit applets in a hypothetical ;) scenario
when someone needs to stop running, say, systemd on, say, a Fedora machine.
No doubt this will expose various shortcomings.
To me, This just seems like a fun experiment to demonstrate busybox's capabilities.
Busybox init is a binary. It can run anything (based on /etc/inittab) incl. other binaries. There's no need for it to run shell scripts (other than /etc/inittab lines which are executed under `sh -c "..."`).