On M1-based Macs, Homebrew no longer installs to /usr/local/ but to /opt/homebrew/
So if you wanted to use this on an M1-based Mac that had installed multimarkdown via brew you would need to change all references of
/usr/local/bin/multimarkdown
to
/opt/homebrew/bin/multimarkdown
I've also mentioned this to Brett Terpstra, because some of his Markdown tools also assume /usr/local/bin/ as the only install location -- which made sense until very recently.