[pjmlp]

It means:

- being documented on https://developers.google.com

- being installed by default with the SDK

- having official tutorials provided by the Android team using Kotlin

- having code samples provided by the Android team using Kotlin

Providing the confidence to many companies out there that Google will not change Android in a way that Kotlin compiled applications would stop working.

[ehsankia]

Right, the title of the article is pretty awful and misleading. The language used at the conference was "Kotlin is now a first-class supported language". First-class supported means it gets all the treatments you mentioned above, and probably better tooling by Google for development.