And I wonder what the author means with "more flexible than TeX". I have never seen (except for emacs) a more flexible software than TeX. Fully programmable (although the programming language is somewhat different than the commonly used ones). And with LuaTeX, you can even use Lua to program the system (like SILE) and you can use LuaTeX as a library to create your own frontend while using the power TeX gives (glue/box model, line breaking algorithm, ...)
