It's not a Linux because consumer devices don't run kernels from kernel.org.

At best it is "based on Linux", or maybe "Linux-compatible".

This is an extremely important distinction, because proprietary kernels and drivers is why you can't install a different Linux distribution on your Samsung or Xiaomi phone.

Yes, but the point was about a "Linux distribution". Linux is a kernel, and a "Linux distribution" is generally understood as Linux + some fairly specific userland (generally GNU).

Of course it is a gradient, but Android goes pretty far away from the common understanding of "Linux distribution".