Is Fuchsia considered to be Unix?
As long as your apps run inside Chrome's runtime, or interact with the Android APIs instead of the low level POSIX APIs, they'll never know the difference. (And we all know that Android's existing POSIX support is minimalistic, to say the least.)
So unless one is coding in straight C there is very little pressure to care about POSIX anyway, as all other languages, including C++, have a better OS independent standard library.