Kind of for a very long while. You then had a descendant SFU from some SP of NT4 to XP / Server 2003, then a further one SUA until Windows 8 / Server 2012. With some code flowing between various companies. I think SFU still used the Posix NT subsystem core. Probably also SUA, although I'm less sure. Not really the case WSL1, though (although probably the core NT kernel was more ready to support it, thanks to its history).