That's only half-true. Understanding all of Lua is hard, specially if you want to get into optimizing your code, but for simple tasks it's an incredibly easy to learn language. That being said, it really depends on the resources you're using to learn; Tutorials meant for programmers that bombard you with slang and exotic concepts will confuse newcomers.

Lua was made specially for non programmers. I think it is really intuitive and easy. I agree with the comment that perhaps understanding optmisation details is hard. But that's hard in every language.