kotlin-test's assertNotNull lets the compiler know that whatever is passed to it is not null afterwards, while junit's assertNotNull does not. Why is it like that?

its probably using contracts. look here https://kotlinlang.org/docs/reference/whatsnew13.html

yes

/** Asserts that the [actual] value is not `null`, with an optional [message]. */
fun <T : Any> assertNotNull(actual: T?, message: String? = null): T {
    contract { returns() implies (actual != null) }
    asserter.assertNotNull(message, actual)
    return actual!!
}

Thanks a lot guys!