How does kotlin gradle dsl generate... I guess the ability to use extensions like tasks.jar?

What do you mean exactly? It uses configuration time information about available tasks/extensions/configurations and generate Kotlin code with type-safe accessors

Well like, if I use apply<SomePlugin>() that doesn't work in a typeasfe way... UNLESS I do it in a subprojects block in a superproject.

Apply doesn't generate accessors, only applying plugin in plugins block