My impression, though I'm not sure, is that Kotlin simply won't allow you to do a double "??". Scala etc. will technically allow Option[Option[Something]], but in practice you would almost never want to use it, and can easily avoid it with flatMap.
The whole point of this was to show that Scala's types preserve the structure of the computation.
It might not be very interesting in the Option[Option[String]] case but imagine Try[Either[String, Int]] or List[Future[Double]].
It's a very important distinction.
Collapsing cases is one of the primary thing why exceptions sometimes get a bad rap, and Kotlin (and Ceylon) do the same with ? (and |, &) at the value level.