Windows had BSD code in their TCP stack for a couple releases of NT. There is still some BSD code in it and you can see the authorship notice if you do a strings on FTP.EXE;
I think Windows Vista got rid of the POSIX part by now. Embrace -> extend -> extinguish -> done.
Having BSD-derived userspace (like ftp.exe and probably more) does not imply having BSD derived TCP stack. And the internal architecture of windows networking is significantly different from any unix (one could probably also call it "broken") so I find it hard to believe that there are major parts of BSD-derived code in there.
Vista is probably first version of Windows where Microsoft actually advertises the POSIX part, so it's there and probably it's here to stay.
I cannot prove Windows had BSD code in its TCP stack as I never had access to its source code, but I never implied it must have been so _because_ ftp.exe has BSD code inside it.
There seem to be numerous references to BSD code inside Windows TCP stack from the late 90s. Maybe it is only utban legend, but maybe it's not.
I think Windows Vista got rid of the POSIX part by now. Embrace -> extend -> extinguish -> done.