That's not actually true, IntelliJ support for Java has always been much better, and still is today! Just try auto-completion in Java VS Kotlin in your IDE... it's much faster in Java files, something I've been complaining about as I've been using more Kotlin than Java, recently. Check also how many refactorings are available in your Java files compared to the Kotlin ones.

Jetbrains knows its main source of income is still, by far, Java, so they don't drop that ball.