I guess some automatism is broken there and noone recognized it yet.

Embedded Kotlin version in those docs is

1.2.41

, so it is older than Gradle 5.0 which is the first mentioned at https://docs.gradle.org/current/userguide/compatibility.html#kotlin

5 years ago that @Paul Merlin updated these: https://github.com/gradle/kotlin-dsl-docs

But still linked to from docs.gradle.org

Too bad the technical writer they had, left after short time