I wonder if the inclusion of Kotlin in a de facto standard library (with its impact in size/methods) would mean that library developers will lean more towards Kotlin now

size is only 7kb as stated in article:

Kotlin is the best language for Android application development. We expect many Android teams to already be using Kotlin. For their projects Kotlin is already among the applications’ dependencies. Those that don’t can use R8 or ProGuard to shrink the Kotlin library dependency. When we measured the net increase was negligible: just 7 KiB.