You remark that “A physical object implementing the state-machine-which-is-us and being in a certain state is what we mean by having a unified mental state.” You can stipulatively define a unified mental state in this way. But this definition is not what I (or most people) mean by “unified mental state”. Science doesn’t currently know why we aren’t (at most) just 86 billion membrane-bound pixels of experience.
You remark that “A physical object implementing the state-machine-which-is-us and being in a certain state is what we mean by having a unified mental state.” You can stipulatively define a unified mental state in this way. But this definition is not what I (or most people) mean by “unified mental state”. Science doesn’t currently know why we aren’t (at most) just 86 billion membrane-bound pixels of experience.
There is nothing else to be meant by that—if someone means something else by that, then it doesn’t exist.