I mean that argument is ridiculous, saying that things are “documented” when the thing is literally called tmpfiles.d and the man page starts with the following explanation:
It is mostly commonly used for volatile and temporary files and directories (such as those located under /run/, /tmp/, /var/tmp/, the API file systems such as /sys/ or /proc/, as well as some other directories below /var/).
So basically some genius decided that its a good idea to reuse this system for creating non-tmp directories. Overall my opinion of systemd is reluctant acceptance though I always wondered why the old way was a problem. Need a service started on boot? Well, we had crontab and sysvinit with some plain files. Need a service shut down? Well that’s the kill command. I guess I don’t really know why systemd was made