Certainly just because you're using the Cygwin version of vi or bash doesn't make a question about regexp replacement or brace expansion off-topic.
When it comes to questions that are more directly tied to a particular implementation, it seems to me that since we allow questions about interactions between Unix implementations and hardware, we should also allow questions about interactions between Unix implementations such as Cygwin, GNUWin32, Windows Subsystem for Linux (Ubuntu on Windows, Bash on Windows), coLinux, etc. and the underlying platform.
However, if a question is essentially on the Windows side and the fact that Cygwin is a Unix implementation is completely incidental (e.g. cygwin1.dll woes), I think it should be asked on a Windows site, because I don't expect that there is much expertise about Windows on this site.
As for this question, the asker doesn't even know what Linux is. I think it would fare better on Super User. I'm not sure that the asker even should have been using Cygwin in the first place.