For me it's mix of Linux kernel and a subset of common FOSS software[0] providing the boot and low-level operations, while the middleware is provided by AOSP with Dalvik/ART and the top, the whole user experience with GUI, apps and whatever. The middle and the top has absolutely nothing common with Linux.
To give you and idea - if you swap the engine in your Ford to Cummins, would you tell everyone what you are driving Cummins? No, you would tell what you are still driving Ford. The same applies to Android, if you swap the Linux kernel to something else, eg to OpenBSD kernel, would you tell what your smartphone is now running on OpenBSD? No, it's still Android, though with OpenBSD kernel.
[0] if you have Linux kernel then it doesn't makes sense to write tooling and userspace from scratch. Like you can, but.. why?
So, it IS a Linux distribution with other userspace that's not made by GNU. What some people call non-GNU/Linux.
Distributions like Alpine for example would also be it.
>The same applies to Android, if you swap the Linux kernel to something else, eg to OpenBSD kernel, would you tell what your smartphone is now running on OpenBSD? No, it's still Android, though with OpenBSD kernel.
Nobody says that it wouldn't, and it also would be true for other Linux distributions. Remember Debian GNU/kFreeBSD?[0]
Now, you could argue that someone who's using the terminal in Debian GNU/kFreeBSD would notice that it's not in fact Linux, but that's a matter of expertise (not every Linux user relies on the terminal), and that's also true for Android.