I've programmed a tiny bit in Lua as it's trivial to embed the interpreter in C, so even beyond gaming in things like Redis it's used as a kind of extension language. Multi-language systems often involve calling routines written in C from more expressive languages for performance reasons. Lua is used for the inverse - calling routines written in more expressive languages from C.

What's nice about Lua is it's purpose built for this (in contrast to something like Ruby, which I've embedded in C very painfully). Writing a game in Lua sounds like a chore though. Reading over the interview, I'm surprised they don't regret it.