That’s not really what that blog post is talking about. Lua isn’t actually particularly old as far as programming languages go and one of the most commonly used scripting languages in game development, due to it’s easy embeddability. And it’s a perfectly fine language in that regard.
Their problem is that they built their own visual scripting language on top of Lua called BlockBuilder. And that comes with quite a bit of overhead, since the way they’re doing it needs a number of additional heavy operations. And Lua is a full blown programming language that comes with a lot of functionality that they don’t need for that use case, but still need to account for.
So the complaint is, that they used Lua instead of using a simpler and constrained language