I never worked with Kotlin, but have kept it in the back of my head as the simpler alternative to Java if I had to target the JVM. What makes it more complex?
Kotlin provides a lot of 'clever' syntactic sugar and features that makes certain things that are quite verbose in Java, nice and compact in Kotlin. It also means that there are many way to achieve the same thing in Kotlin. Once you've learned everything it allows you to do things that would take a lot of Java in much fewer lines of code. But there is also just more 'stuff' to learn in the Kotlin language.
Java is a much simpler language than Kotlin with relatively few features, but this simplicity means you sometimes have to build quite verbose structures to achieve what you want.
So which is 'simpler' very much comes down to how you define 'simple'.
Kotlin's one big advantage is having avoided the "billion dollar mistake" of null pointers. Or at least, mitigated it. But in my opinion it's not sufficient to install a whole new language into your stack.
I believe I read on the mailing list it’s implemented here: https://github.com/openjdk/valhalla/tree/lw5
I also think that checked exceptions are a major flaw (particularly because they don't work well with lambdas) but I don't foresee them getting removed anytime soon.
Kotlin is incredibly expressive and can get quite complicated. Java is a bit more brutally simple (and obviously more verbose).
As a C# dev primarily, I love Kotlin, and I've always hated Java despite it being the very first language I learned.
I wouldn't call Kotlin simple by any stretch of the imagination. It's just a very high-level language.