this has had a slightly surprising response on twitter:
It seems that they don't want the stdlib as a transitive dependency, as other kotlin libs may require a different version? ... and they don't want to go down that route.
... and they're worried that Kotlin types would appear in the API making it unusable from Java.
it'll be interesting to see how this plays out, it's an extremely popular library. I'm struggling to think of any other Kotlin libs with this level of popularity (except stdlib).
Personally I think this is a great move.