In theory, sure, but there's no way Hare is going to ever officially support Windows. Even if the compiled backend supports the target, the language still needs to abstract over the OS interfaces and implement syscalls.
The Linux kernel ABI, while stable, is not simpler than libc and not portable at all. You can build the modern computing environment on mingw.
However, if they chose to target Linux–only to show how low–level Hare is, that's understandable as well —no shade.
To take it to its logical conclusion, they're saying libc will be a package you install on your Hare/Linux system for compatibility with obsolete systems, which to me implies willingness to work on ports to other platforms —you're not going to replace libc otherwise.
The main issue on that OS is that it is neither ABI or API stable.