No, I don't think it's OS sniffing since downloading the windows version of for example Filezilla doesn't allow you to change mirror and as the article explained, downloads from some other url than Sourceforge (or a mirror).

I'm on OSX and when downloading the windows version from the repository, I get the same file as the author. Try it yourself here and download the .exe: http://sourceforge.net/projects/filezilla/files/FileZilla_Cl...