I was including Linux there, as it follows a standard UNIX architecture.
There is a big difference in having POSIX support and being a UNIX.
Windows, IBM i, IBM z/OS, Unysis MCA ClearPath, Green Hills INTEGRITY OS, Genode and many other OSes have POSIX support, yet their architectures have nothing to do with UNIX.
UNIX is married with C by design, just how browsers are with JavaScript.
Actually you are right in one thing, if Redox offers POSIX support then it needs to sandbox them.