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.)

Fun things is that I encountered that for the first time when using Clozure CL (https://ccl.clozure.com/) which quotes colons when converting paths to string even on Linux:

  $ 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

Current macOS Finder let's you name files with a slash in them, rendered as a : in the terminal.