I doubt anybody really writes 'POSIX' anymore, if that helps.
What they do, more typically, is 'Write Linux software'. If they care about POSIX they will try to ignore features they don't think was mentioned in some POSIX manual from 20 years ago. It's all very hand-wavy.
This is why you see the major operating systems advertise Linux software compatibility rather then paying for POSIX certifications. AIX, Solaris, Windows, etc. Sure it's not a official standard, but it's going to be pretty well defined because you can just model your compatibility on WWLD.
If push comes to shove then Fuchsia could just add some variation of 'usermode linux' as one of those userspace kernel services.
I've the feeling that POSIX is an API that makes many assumptions about how things internally are implemented in the kernel. So POSIX as a library is often limited or inefficient.