This doesn't clarify much.

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.