It is, but userspace isn't aware of it.

For userspace what matters is ART, ISO C, ISO C++ and Android NDK APIs.

It uses a (slightly modified) Linux kernel, yes, although its user land is decidedly not GNU or particularly Unix-like.

Doesn't each phone basically ship its own modified Android-Linux with hacked up drivers specific to the model?

Ever since Project Treble it got standardized.

I thought Treble just made the hacked up drivers use a standard ABI so you could combine them with a different userspace?

Yes.