Earlier quoted context omitted.
>which is the reason there are very few phones that can run actually run mainline (real) Linux. You mean the drivers not being in the Linux Kernel/hard to port ? because I have a very hard time believing most smartphones couldn't run the Linux kernel, provided it's compiled to whatever architecture and the drivers are there.
Android Linux is quite different from upstream Linux, specially after Project Treble. Now it looks more like a pseudo-mikrokernel, with drivers running on their own processes, talking via Android IPC with the kernel layer. Also it has a much more security knobs turned on as mainline, and many features trimmed down that are irrelevant to Android userspace.
If anything, Android 8 reduced the gap to mainline with switching e.g. to DRM synchronization fences.