Old Macs (which I grew up with) had even more baroque path handling than mentioned in the blog post:

Double colon (::) meant the same as .. on Unix/DOS, that is "go up one level". So you have to be careful when concatenating paths to not get double separators.

Paths starting with : were relative. If a path didn't start with the separator, the first component was the volume name (disk partition). Again, quite unlike Unix.

Also, remember it was common to have spaces in names on Mac, even the default harddrive on Macs was named "Macintosh HD". So an absolute path like "Macintosh HD:Programs:MacWrite" would have been common. (I grew up with Macs in Swedish, so I'm back translating the names here, could be that the names were slightly different in English.)

  $ cat <<'EOF' >x.lisp
  heredoc> (require :uiop)
  heredoc> (let ((p (make-pathname :name "foo:bar")))
  heredoc>   (format t "~@{~A~%~}" (namestring p) (uiop:native-namestring p)))
  heredoc> EOF
  $ ccl -b -Q -l x.lisp </dev/null
  foo\:bar
  foo:bar
  $ sbcl --script x.lisp
  foo:bar
  foo:bar

    $ touch "foo:bar"

It took me a long time to understand why colon wasn't a valid character for file names on Mac and I still find the colon separator to be the least visible these days. Finder can display paths with the forward slash separator (defaults write com.apple.finder _FXShowPosixPathInTitle -bool YES), and yet forward slash may be used in a file name created through Finder as noted in the post, while colon cannot (which is not addressed), but creating a file in the terminal named with a colon is possible and the shell will escape it correctly in use. This file then shows up with a slash in place of the colon when viewed in Finder, and conversely the file with a slash in the name shows up in Terminal with a colon!

I was expecting the story of the magical ¥ path separator

Now imagine operating system, that has no directories (and no path separators) or no filesystem at all.