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.
I'm not familiar with the language but if it targets libc, it might be easily able to work with MINGW. There are mingw/native versions of most popular libraries.
The whole point of Hare is to be a part of a modern computing environment that offers most of the creature comforts of modern computing, yet is small enough to be understood by one person.
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.