I think there is more to Actors than just single-thread-access. Namely only the code of a given actor(-class) should be able to read and write the internal stat

I think there is more to Actors than just single-thread-access. Namely only the code of a given actor(-class) should be able to read and write the internal state of "instances" of that class. Why because then only the code-unit/module that defined the state variables can depends on their definition. A bit like "objects" internals can only be accessed by methods of its class.