That may be the history, but they are now completely separate. The native windows mode of cygwin really is taken from the mingw project, not vice versa. If you ask questions about it on the cygwin lists, you'll be told to buzz off (
, that is, you'll be politely redirected to the mingw people). And there's no reason to just install multiple megabytes of mingw when I already have the tools to do the job.