Others have said most of it, but as an addition, here's an example project that has a Kotlin/JVM backend (Ktor), and a Kotlin/JS frontend (React)
https://gitlab.com/clovis-ai/formulaide
It uses code sharing for all data structures in the API (eg. for validation) so the web-based UI can validate input with the exact same code as the backend.
When they say ‘Kotlin Multiplatform is in Alpha' that more or less means the gradle config is in alpha. The language itself (expect actual) is very unlikely to change (or if it does, you can expect very good migration guides because
everyone is using those. The most likely thing to break is an update to the gradle plugins that requires a different configuration—but that's not going to kill your project nor require much effort to migrate.
The example I gave only has a Kotlin/JS frontend, but it would be trivial to add JVM and native support for all parts (the only exception is the UI, there are currently no good multiplatform UI libraries, but JetBrains Compose is on the way to fill that role one day)—that means that with a few lines of gradle & CI config, essentially a few hours of work at the maximum, I can have all the business logic, validation, data structures, etc in any other platform.