I believe that "supported" here means the test suites and CI are run on those platforms. More importantly, any regressions on those platforms would be considered bugs. However, there have been partial or complete ports to several other platforms, including some weird ones.
Aside from Apple platforms, I see code in-tree for Linux, CYGWIN, Windows, FreeBSD, PS4[0], Android, and Haiku[1] (a revived BeOS). And that's not some stray LLVM code, it's in the Swift compiler itself. I've read that there was/is an out-of-tree upstream port to some IBM mainframe hardware, as well.
A bit tangential to your original point, but I think it's interesting.
[0]https://github.com/apple/swift/commit/83901998c91f9242a133aa... [1]https://github.com/apple/swift/commit/aee81d272f3147c0a9b610...
”macOS, Ubuntu Linux LTS, and the latest Ubuntu Linux release are the current supported host development operating systems”
Also, on the page you reference:
”Note that all compiled Swift binaries are only executable within Bash on Windows and are Ubuntu, not Windows, executables.”