But surely it's not named "POSIX mode" for no reason. It's because those rules resemble those of the UNIX shell. While "non-POSIX mode" resemble those of non-POSIX shells, such as DOS.

shlex.split seemed to be a shortcut for those wanting to simply parse a generic quoted string who weren't interested in creating a minilanguage. Surely it should be possible to avoid POSIX rules when doing this on Windows?

You haven't suggested any other way to do this. The fact is, I do want to parse a Windows command line. The only way I have found is by copying shlex.split and hacking it. Didn't seem very nice, especially it seems a fix would be totally trivial, but it's obviously better than starting from scratch.