I think you may have misunderstood what you're looking at. Julia is very much a dynamic language. It also has the ability to add type hints, but, unlike for most optionally typed languages, these aren't just type hints for human readers or an external type checker. They're actually one of Julia's more interesting features. Julia's type declarations provide information that the compiler uses to generate faster code, and to support its multiple dispatch functionality.
When the notes say "optional typing - currently not implemented", all they're saying is that this syntax doesn't allow you to tap into that feature, because a syntax for type declarations has not been provided.
Just a minor point - type hints only really help if you put them in struct fields. In function signatures they only limit what can be dispatched to that specific method, there's (almost always) no difference to leaving it untyped since functions are compiled for specialized types anyway.
When the notes say "optional typing - currently not implemented", all they're saying is that this syntax doesn't allow you to tap into that feature, because a syntax for type declarations has not been provided.