The majority of APIs are not exposed to NDK users, only to OEMs.
Google could release lets say Android 8 with another POSIX compliant kernel and the only apps that would notice are the ones using non official APIs.
Currently, Android is released using Linux kernel, ELF executable format, POSIX API's, and so on. There is no Android/kFreeBSD, nor Android/NT.
Android games can be launched on Linux using android libraries (not all, but some works pretty well, see: http://www.shashlik.io/showcases/ ).
Linux tools can be launched on Android systems (including X based tools, if X server is running).
For most of practical purposes, Android is Linux.
Debian user space isn't the same thing as a kernel.
Android kernel doesn't expose the same syscalls as a standard Linux.
I am really keen in having Google replacing Linux with Magenta, then we can carry on this discussion about what Android is supposed to be.
The majority of APIs are not exposed to NDK users, only to OEMs.
Google could release lets say Android 8 with another POSIX compliant kernel and the only apps that would notice are the ones using non official APIs.