Pretty fascinating stuff. I wasn't aware of the push to do some static annotations in Lua and that seems like a great idea.
I've used at least three bespoke configuration languages where I eventually had to ask "Why isn't this just Lua?" I highly recommend embedding Lua as the first option over writing one's own config language and parser any time someone suggests it.
I wonder why the author doesn't use Teal [0] - a typed dialect of lua.