Yeah Lua optimizes array tables for performance as long as they're arrays. That's the only optimization I'm aware of. I get why they didn't just add arrays, to keep the syntax and semantics clean, simple, and unambiguous. I just don't like it. If you take that to its extreme, you get Lisp. Natural human languages are messy and full of warts, but they work despite that, or perhaps because of that, because human life is messy and warty. "Pure" languages never can or do catch on. My favorite language right now is unironically TypeScript despite all its baggage.
More than just keeping it simple, it's also about reducing the API surface of the language. Lua's main design constraint is that it's meant to be embedded inside other applications, and the versatile table type helps a lot in that regard.