OS X, yes it is a certified UNIX, but the software world is more than command line tools and UNIX daemons.
Any OS X, iOS, tvOS, iOS application that regular users actually want to use, rely in Objective-C APIs making use of classes, categories and protocols, with some of them actually being nowadays implemented in Swift, like the new audio stack.
ChromeOS, yes it is built on the Linux kernel, however the whole stack is built to manage Chrome instances. Only web apps count. Any developer creating applications to ChromeOS only deals with the web stack.
Android, oh sure it is built on top of a forked Linux kernel, stripped out of any signs of UNIX V IPC, but regular application developers only see Java APIs. Surely there is the NDK, which exposes about 5% of Android APIs, has a very limited set of official C and C++, and any attempt to link to on-device libraries not part of the sanctioned list, terminates the application, as of Android 7.
Windows, sure the kernel is built on a mix of C and C++, both Win32 and UWP build on top of that. However .NET Native is also able to produce UWP components, just like C++/CX. Watch any Microsoft conference since the launch of Windows 10, and you will see in which direction the wind is blowing on the whole .NET Native vs C++/CX for UWP applications.
Tizen, yep they built it on top of Linux kernel, offer some native API that has already gone through three reboots, but guess what. The main APIs are their web stack, native is not even allowed on Tizen for TVs and now their are adopting .NET Core as the new API stack.
As for POSIX and mobile OS, maybe you haven't been paying attention to the news.
"POSIX has become outdated" -- http://www.cs.columbia.edu/~vatlidak/resources/POSIXmagazine...