I mean, the technical limitations are pretty obvious if you are even remotely familiar with the architecture. The watch offloads significant computation and communication to the phone in an extremely tightly integrated manner to have an acceptable computational and power consumption envelope.

I don't see how they could have done it without significant co-operation from the Android OS team far deeper than even two allied companies are typically able to communicate. The other alternative would have been to design a totally different piece of hardware that operates according to the Android OS expectations but that's hardly "bringing Apple Watch to Android".