Android is using Linux, but calling it Linux is entirely wrong.

It tries very hard to hide any non-portable components from you. The base platform is a virtual machine where you have zero access to any Linux API. They even indent their own API for things Linux already has! (Binder, ashmem, power management...) About the one Linux API you have access to is sockets, if you're writing native code, and there mostly a POSIX subset. (No, you do not really have access to file system directly.)

It could run on Minix or Windows or Darwin for all Google cares about - but hardware manufacturers preferred Linux for the price.