Not really, you missed the point that the Linux kernel is actually irrelevant for app developers on Android, to the point that ART might in the future run on top of Fuchsia and no one would notice.
There are no Linux kernel specific APIs in the list of stable NDK APIs.
You might naturally create an NDK application and do a bunch of syscalls, just it doesn't mean that the APK is guaranteed to work across all Android devices.
Of course Linux is an implementation detail. When did I claim otherwise? That actually proves my point: Fuschia is another implementation detail as well.
There are no Linux kernel specific APIs in the list of stable NDK APIs.
https://developer.android.com/ndk/guides/stable_apis
As of Android 7 using non-stable APIs will kill the APK on launch.
https://developer.android.com/about/versions/nougat/android-...
https://android-developers.googleblog.com/2016/06/improving-...
https://android-developers.googleblog.com/2016/06/android-ch...
Only OEMs get to deal directly with Linux kernel.
You might naturally create an NDK application and do a bunch of syscalls, just it doesn't mean that the APK is guaranteed to work across all Android devices.