The difference is that self-documenting code is bad. Code isn't documentation for humans. Documentation is documentation for humans.

One comment right above this that gives a direct example of what x "is", is easier to understand. And then the structure of this line is also easier to understand more quickly than the second example.

Is this bad?

  // A tuple defining an item with a status. (item-name (status-key status-value))
  val x: Pair<String, Pair<String, String>>

But even knowing what x "is" either with documentation or with verbose "self-documenting code" it still doesn't answer your question of "how this code should be used in the context of the application"

Do we save x somewhere? Do we fetch something from somewhere else based on status-key? None of this is known. You get that from description, you get that from surrounding context. Except if we write all of our code like the second example then when we start reading the surrounding context it's going to be more painful.

We're hiding the structure with extra verbosity in the pursuit of not having to write documentation or use the best tool we have for communicating with other humans... natural language.