Android devices are based on the Linux kernel but, since the beginning, those devices have not run mainline kernels. The amount of out-of-tree code shipped on those devices has been seen as a problem for most of this time, and significant resources have been dedicated to reducing it. At the 2018 Linux Plumbers Conference, Sandeep Patil talked about this problem and what is being done to address it. The dream of running mainline kernels on Android devices has not yet been achieved, but it may be closer than many people think.
As I always say – Android is the most popular Linux distribution of all time, and by a huge, huge margin. This often makes a lot of people very angry, as they come up with all sorts of additional random imaginary requirements as to what constitutes a Linux distribution. If they manage to get the Android version of the Linux kernel back into mainline, these discussions will probably become even more nebulous – and entertaining.