Objects and "method calls" generally implies mutable state to me, but yes the parent was not explicit about this. I assumed mutable (implicit) state was being argued in favour of an explicit state representation. Perhaps I misunderstood.

For a state machine, I would expect a function such as:

transition : (old_state, event) -> new_state

Or if we use immutable objects, and one method per simple event, then something like:

transition_event1 : () -> new_state

Which I think is similar to what you hace. So I think we are in agreement here.