You can use kotlin for more things than just Android. And before it had official support from JetBrains (for Android among others). The way it works in Android doesn't make any difference with other popular libraries as retrolambda, picasso, dagger, etc. Those aren't supported by Google and still used in a lot of Android apps.

Google only said they are going to collaborate more with JetBrains, which in my mind it translates in "we, developers, are not going to have incompatibility issues between kotlin and beta versions of the gradle plugin". (I want to highlight the word beta, because with stable versions it was weird you have any issue with kotlin).