Apple releases a great deal of
open source software, which, so far as I'm aware, all runs on Linux as well. At least Swift, clang, and LLVM, all run on Windows as well. So does their Objective C compiler, so of Apple's
programming languages, that leaves AppleScript. I would not describe AppleScript as robustly successful.
I believe Apple could probably get away with keeping Swift proprietary, or only supporting Apple platforms. But they don't. I have no inside-track information on why that is, but I suspect the reason is fairly simple: developers wouldn't like it.