Was that ever verified? I know that detractors said that, but MS claimed that the copyright notice was only there to cover the utilities like ping.exe and finger.exe (which really were based on BSD).
I think the older NT source code is now available to read, isn't it? Going back and sorting out the truth of some of the claims from the 90s OS wars might actually be quite a fun research project...
I'm not sure if the NT IP stack was ever BSD, but many of the windows 3.11 and early 95 (before Microsoft natively added support) were from the BSD stack, or at the very least re-implemented the BSD socket API (Most famously Trumpet Winsock). This probably fed the rumours. IIRC winsock was also bundled as an "extras" in some pre-native IP supported versions of NT.
I think the older NT source code is now available to read, isn't it? Going back and sorting out the truth of some of the claims from the 90s OS wars might actually be quite a fun research project...