This library seems to add a HTTP client impl built on top of NSURL, and another on top of CURL. It looks to me like everything else here is provided by Microsoft's CPPRestSDK - WinHTTP, UWP & Boost Asio based clients simply use the CppRestSDK impls under the hood.
It says winhttp is used on windows...
Edit: Sorry, I misread your post. To add, cpprestsdk has 2 impls for Windows - one based on top of WinHTTP and the other based on WinRT APIs. On UWP, cpprestsdk uses the WinRT APIs and by extension so does NFHTTP.