The kernel is less of a 'Linux distribution' than that. It is just the basic piece of software that manages the resources of the system and provides them to the operating system above, which in our case is Mer/sailfish.

The reason the Jollas must use Android Linux kernels is that the hardware it uses only has Android drivers available. The manufacturers don't give Jolla the source code for their hardware drivers, only binary drivers for a specific version of an Android kernel. Stskeeps came up with a translation layer that lets normal Linux software work on top of an Android kernel, which let Jolla assemble lots of standard Linux components into a real mobile Linux operating system. This is Mer, and then with the gesture OS and silica look and feel on top of that, Sailfish.

Alien Dalvik lets Android apps sit on top of Mer, and simulates the rest of Android for them. For instance, Whatsapp is given a view on the Sailfish/Mer address book, as if it were the Android one.