What's interesting here, is that they seem to have actually started with Lua, but then switched to a custom language at some point. I'm not quite sure what were the exact reasons, as googling leads to a github wiki which since "has moved", but there's some discussion in an earlier HN submission [1]. That said, personally I can't say I'm really convinced (although I'm not a game programmer by any means).

[1] https://news.ycombinator.com/item?id=7209149