But is that different from using Kotlin on iOS?

kotlin has kotlin multiplatform. A project to run non-ui components on both platforms. It's been there for a few years, apple hasn't even started.

You can run non-UI Swift on Android too if you want. I don't know who made that possible, but I also can't see why Apple would sponsor Android app development, seems completely counter to their interests.

you can in theory, but the binding with the jni world will be atrocious.

You're also pretty much on your own with the library ecosystem.

I agree that it's not apple's interest. But that's part if the problem : this language didn't start with the goal of being just an apple language.