Understood, but could you not do this by coding style? I.e. calling a method returns void?

Whether something is already implemented in a language's core library, or a package you can use, or you build your own library, is immaterial. You can implement any patter with any turing complete language, and you can build your own language.

That's true, but I'm not really talking about that.

Correct me if I'm wrong, but you asked if message passing rests on language facilities or code style. Answer is neither. You can implement message passing patterns in Forth, in C, in Python, or you can use Erlang or Smalltalk and get it out of the box.

I wasn't making it exclusive. I think the answer is either, not neither.

The thing I wasn't asking was "can I implement a language that can do this in Java?" which is why I don't think the Turing completeness comment is particularly relevant.

Depends on neither is the same as 'you can do with either'. Just different ways to express the same answer.