absolutely irrelevant that in the backend is running a linux kernel

I don't think so. Android is a Linux distribution, because it distributes Linux and is build the operating system using it. Besides that, Linux itself isn't fully POSIX standard conform too: https://en.wikipedia.org/wiki/POSIX#Partially_conformant and thus no operating system built distributing the Linux Kernel is POSIX compliant. Android supports lot of POSIX functions, so its not that far off. The difference is not POSIX, it's actually GNU tools and proprietary blobs included, and the user application packages that make it so much different.

Also interestingly there is a man page about these type of standards: https://www.man7.org/linux/man-pages/man7/standards.7.html