This reminds me of the classic results showing Turing completeness of things like sendmail.cf and CSS+HTML. The trick of using directory nesting depth as a counter is clever — it essentially turns the filesystem into a tape. I wonder if there is a practical upper bound from filesystem limits (e.g. PATH_MAX) that would make this more like a bounded automaton in practice.
They explicitly state that using `-execdir` to change the working directory avoids issues with PATH_MAX; though I didn't see any mention of the working directory itself having a limit (which I assume it does, for Linux processes?)
