Android is one platform: android. I thought rust worked across multiple operating systems.

Operating systems, not architectures. You'd have to cross-compile your application 4 times if you want to support all arms and x86s.

I think the main benefit of rust/c++/ndk on android is that I can just port desktop programs and I don't have to learn android's java/kotlin and sdk.

It's not "just" port desktop programs. Android doesn't even use glibc.