I also have some confusion in the same file, where I see:

plugins {
    kotlin("jvm") version "1.8.0"
}
The version should be literal in this block, and it cannot be applied from another build script.

I interpret the restriction to mean that one cannot apply the Kotlin Gradle plugin from a custom plugin. Is that a correct reading? I sure hope not.

Sounds pretty arbitrary. o_O Besides that no version should be literal anywhere, except in version catalogs. :-D