Sailfish does not run on top of Android. It is a gnu/linux system with a Qt interface. It has an Android layer called AlienDalvik which runs on top of it. But it also has native apps written in C++/Qt. You can also run FlatPack apps which is interesting. I believe it uses Wayland for it's display server.

The Android apps blend pretty seamlessly with the whole experience. For example, the keyboard that pops up in Android apps is the same as the one in that pops up in native apps.

My guess is that Sailfish OS is reusing android device drivers.

I don't think so, the model numbers are specified:

> Sony Xperia X Single SIM (F5121) > Sony Xperia X Dual SIM (F5122)

Get the compact model's number to check.