yeah, in theory this is "just software"

but in practice it is very much about control. Watch apps are really split with part of the app living in the phone, with proprietary efficient state and communication between phone and watch to support the functionality.

Maybe apple does not want to support Android for other reasons, I cannot say. But at a technical level it would be enormous work to support android, and it would require allowing apple to tweak the low level comm stack to support the same functionality as the watch has today. Apple does not have that kind of control over the Android stack.