JVM can run on any platform that supports Java, as you’re building Kotlin directly into Java bytecode.
Multiplatform is for building native applications while using a single backend logic. You’ll have to write separate handlers for everything unique to the platform, according to documentation.
I think the reason they’d need to do a thing in a pressed timeframe is that, while psychohistory describes populations over time, sufficient understanding can allow someone to make a relatively small tweak that ripples through time.