If userspace needs to use NDK/JNI ABI to call the Linux C ABI, naturally the OS ABI isn't the C ABI, by definition.

Why not? The NDK/JNI calls are still in user space themselves. So what delineation are you trying to make here?

	▲	pjmlp 2 days ago \| parent [-]
		How userspace applications talk to the kernel subsystems, in a legal way without hacking the operating system architecture.