I think it's pretty clear. They want an OS that can scale to any device. Whether Fuchsia replaces Android is unclear, but having their own PC OS in which Android can seamlessly integrate with much like iOS and MacOS can is very appealing.
Integration between iOS and Mac OS may just be because Apple controls both of them tightly. It doesn't tell us much about OS similarity or any deeply shared code. You can make two very different OSes integrate seamlessly with each other given full control over the OS code.
