When you're booted in the installer environment, /etc is writable. Some of the sample code in the article shows writing a file in /etc, and the file write works fine, mdadm just doesn't read the file.

I believe the reason is that mdadm in the installer exists to create arrays for the installed system, so the installer's RAID configuration needs to supply a configuration file, and for some reason that's handled by having the installer's RAID configuration write it to /tmp/mdadm.conf and having mdadm read it from there. (Rather than, say, writing it to a temporary file and directing mdadm to read that temporary file.)