Maybe I'm misunderstanding, but this doesn't sound all that different from your average static site generator—Python-based Jinja templating for HTML files versus Ruby-based Liquid templating for HTML/Markdown files (i.e., Jekyll).
And that's cool. Making stuff is fun.
Maybe I'm being obtuse, but the reasons they gave for not using an established SSG primed me to expect a radically new solution. Versus "we didn't like any existing SSGs so we built our own," which was my takeaway from the whole thing.
A PHP site (for the includes), or an out-of-the-box SSG would make much more sense, IMO.