@tigerroast & @endsormeans nailed it pretty much down, but I sill want to give my 2c on top of that.

Indeed, Android has different library collection than your standard GNU environment, which leads to pretty different userland. Android kernel is full Linux kernel though, so you can say that it is a sort of Linux distribution, only very far from your standard distribution models.

You can port a lot of existing CLI software to it with (mostly) minimal effort though, there exists a wrapper thingy for that in the Android SDK. However anything that would like to run in X is not usable.

The limitation of compiling&running X applications is also present on SFOS, but that differs again from Android as it is a true GNU userland distribution where all things CLI compile almost without modifications. Running X applications would be possible on SFOS with some tweaking as has already been demonstarted by @javispedro.