Can I create a (edit: Native) MacOS app without writing a single line of Swift? If yes, is there a sample somewhere?

Sure! You can use the JVM on MacOS. For the UI, you'll need a framework that also works on the JVM. For instance, Compose for Desktop.

Yes, Native is the goal

I can't see why you couldn't avoid swift, however you still need xcode (whole ide, not just cmd line tools) installed to build, sadly...

binaries {
  executable {
    entryPoint = "com.example.main"
  }
}

was what I was looking for. Thanks all!

Have done this with iOS before... Full

.kt

main function, zero Swift. I felt like a King doing it.