I'm not sure it's markdown specifically, seems to be a homegrown markup language, made by Joe himself. Here is the "renderer" part of the website: https://joearms.github.io/src/expander.js

> expander.js - a markup parser

> Hopefully the last JS I every write

> Very few regular expressions were used in the construction of this program

> No libraries were were used in the construction of this program

> All faults are mine and mine alone

> Appears to work :-)

Points to https://github.com/joearms/bookmaker which doesn't seem to exist anymore.