Linux 0.99 was released to the world with TCP/IP stack back in Sept 1992 whereas at the time Windows NT was still in the development since 3.1 version with the adopted Spider TCP/IP stack was only released in July 1993 [1],[2].
There's a time window (pun intended) between the public released of the Linux kernel with working TCP/IP and the official public released of Windows NT. If Microsoft wanted to adopt Linux TCP/IP (I doubt they wanted to do that since it's still at the time pre stable 1.0) theoretically they can because it will be just copy paste exercise as they most probably did with Spider TCP/IP stack. But what I am saying is that even if they wanted to it will be illegal because by the stroke of Linus' genius Linux kernel was relicensed to GPL in the exact same year 0.99 kernel was released.
[1] Linux kernel version history:
https://en.wikipedia.org/wiki/Linux_kernel_version_history
[2] Windows NT: