Android doesn't expose a proper Linux to userspace and Google has been clapping down what we are allowed to do with the NDK since Android 7.0 (edit: corrected, only started in 7.0).
So playing "what if", you can replace Linux with something else that is compatible the NDK APIs and no one would notice, other than the OEMs.
Would you have a link to more details/reading on this? I wasn't aware.
https://developer.android.com/ndk/guides/stable_apis.html
Here are the documentation entries related to clamping down unauthorized use of other on-device libraries or Linux Syscalls.
https://android-developers.googleblog.com/2016/06/android-ch...
https://android-developers.googleblog.com/2016/06/improving-...
https://android-developers.googleblog.com/2017/07/seccomp-fi...
https://developer.android.com/about/versions/nougat/android-...
https://developer.android.com/about/versions/oreo/android-8....
Small correction, it is actually Android 7.0, not 6.0
Makes sense. That way there isn't a massive stockpile of old useless Android devices when the "new Google OS" comes out.