Also, by your definition, Android and Slackware Linux are the same "Operating System" because they share the same kernel and drivers.
Which is exactly my point. This term is in common use and means different thing to different people.
Well, no, they don't. Android still uses a slightly modified Linux kernel, and there is zero overlap in the drivers. Also they have fundamentally different approaches to task & memory management at the kernel level.
Pedantics aside it's clear that Avatar is not an OS, not even with the most generous and broad definition. At least not with what little they've said so far. Because what they have so far in their diagrams and minimal technical info is yet another JavaScript framework paired with yet another set of web services around user authentication.
The Android Mainlining project continues to trudge onward, it's not complete. There's a decent chunk of stuff sitting in staging as well that hasn't been accepted by mainline, but still more that hasn't even gotten to staging. There's also a few things where mainline rejected Google's version and provided an alternative, but Android was never changed to use the new thing (Alarms/Timers fall into this case)