Very interesting. I've contemplated the idea of having a high-level language compile to C as a target language before, but never seen a complete system that does it. It's certainly a useful idea if you want to create your own language generally (not just Lisp-based things). As the article points out, C compilers/libraries exist for just about every platform out there, so by compiling to C your language is immediately cross-platform (assuming you only need cross-platform libraries) and gets the benefit of very mature optimizers, code generators and suchlike.
As for Lua, uh, what? Of course, you could write a Lua-to-C compiler too if you liked.