Windows has been POSIX compliant for about 20 years.
Modern Windows is decidedly not POSIX-compliant.
Anyway, I sense an argument drift from "Windows" to "Modern Windows". Nice. I guess earlier versions of Windows stopped being Windows once version 8.1 was released.
Well, yeah. We're (presumably) talking about software architecture choices here. Are you telling me you'd purposefully build a new system on top of an old version of Windows, and specifically on old features that have since been deprecated+removed? If not, then it doesn't matter what old Windows does; "Windows is not POSIX-compliant" is the fact that you are making a choice based upon when you design a system right now.
BTW, I can purchase a copy of windows 8 today (which is going to be supported by Microsoft till 2023) that is POSIX compliant. So the argument is moot anyway.